diff --git a/reclass/core.py b/reclass/core.py
index d3d6187..ed5a392 100644
--- a/reclass/core.py
+++ b/reclass/core.py
@@ -188,7 +188,7 @@
                             node.interpolate_single_export(q)
                         except InterpolationError as e:
                             e.nodename = nodename
-                            raise InvQueryError(q.contents(), e, context=p, uri=q.uri())
+                            raise InvQueryError(q.contents(), e, context=p, uri=q.uri)
                 inventory[nodename] = node.exports.as_dict()
         return inventory
 
diff --git a/reclass/datatypes/entity.py b/reclass/datatypes/entity.py
index 38360b5..3c927c3 100644
--- a/reclass/datatypes/entity.py
+++ b/reclass/datatypes/entity.py
@@ -83,6 +83,7 @@
         self._exports.merge(other._exports)
         self._name = other.name
         self._uri = other.uri
+        self._parameters._uri = other.uri
         if other.environment != None:
             self._environment = other.environment
 
diff --git a/reclass/datatypes/parameters.py b/reclass/datatypes/parameters.py
index e58a1b1..34ccdeb 100644
--- a/reclass/datatypes/parameters.py
+++ b/reclass/datatypes/parameters.py
@@ -27,7 +27,34 @@
 from reclass.utils.dictpath import DictPath
 from reclass.values.value import Value
 from reclass.values.valuelist import ValueList
-from reclass.errors import InfiniteRecursionError, ResolveError, ResolveErrorList, InterpolationError, ParseError, BadReferencesError
+from reclass.errors import InfiniteRecursionError, ResolveError, ResolveErrorList, InterpolationError, BadReferencesError
+
+class ParameterDict(dict):
+    def __init__(self, *args, **kwargs):
+        self._uri = kwargs.pop('uri', None)
+        dict.__init__(self, *args, **kwargs)
+
+    @property
+    def uri(self):
+        return self._uri
+
+    @uri.setter
+    def uri(self, uri):
+        self._uri = uri
+
+
+class ParameterList(list):
+    def __init__(self, *args, **kwargs):
+        self._uri = kwargs.pop('uri', None)
+        list.__init__(self, *args, **kwargs)
+
+    @property
+    def uri(self):
+        return self._uri
+
+    @uri.setter
+    def uri(self, uri):
+        self._uri = uri
 
 
 class Parameters(object):
@@ -117,10 +144,10 @@
                 raise
 
     def _wrap_list(self, source, path):
-        return [ self._wrap_value(v, path.new_subpath(k)) for (k, v) in enumerate(source) ]
+        return ParameterList([ self._wrap_value(v, path.new_subpath(k)) for (k, v) in enumerate(source) ], uri=self._uri)
 
     def _wrap_dict(self, source, path):
-        return { k: self._wrap_value(v, path.new_subpath(k)) for (k, v) in iteritems(source) }
+        return ParameterDict({ k: self._wrap_value(v, path.new_subpath(k)) for (k, v) in iteritems(source) }, uri=self._uri)
 
     def _update_value(self, cur, new):
         if isinstance(cur, Value):
@@ -128,14 +155,20 @@
         elif isinstance(cur, ValueList):
             values = cur
         else:
-            values = ValueList(Value(cur, self._settings, self._uri), self._settings)
+            uri = self._uri
+            if isinstance(cur, (ParameterDict, ParameterList)):
+                uri = cur.uri
+            values = ValueList(Value(cur, self._settings, uri), self._settings)
 
         if isinstance(new, Value):
             values.append(new)
         elif isinstance(new, ValueList):
             values.extend(new)
         else:
-            values.append(Value(new, self._settings, self._uri, parse_string=self._parse_strings))
+            uri = self._uri
+            if isinstance(new, (ParameterDict, ParameterList)):
+                uri = new.uri
+            values.append(Value(new, self._settings, uri, parse_string=self._parse_strings))
 
         return values
 
@@ -167,6 +200,7 @@
                 ret[key.lstrip(self._settings.dict_key_override_prefix)] = newvalue
             else:
                 ret[key] = self._merge_recurse(ret.get(key), newvalue, path.new_subpath(key))
+
         return ret
 
     def _merge_recurse(self, cur, new, path=None):
@@ -213,7 +247,7 @@
         if isinstance(other, dict):
             wrapped = self._wrap_dict(other, DictPath(self._settings.delimiter))
         elif isinstance(other, self.__class__):
-            wrapped = self._wrap_dict(other._base, DictPath(self._settings.delimiter))
+            wrapped = other._wrap_dict(other._base, DictPath(self._settings.delimiter))
         else:
             raise TypeError('Cannot merge %s objects into %s' % (type(other),
                             self.__class__.__name__))
@@ -224,6 +258,7 @@
                 if value.is_complex():
                     p = path.new_subpath(key)
                     self._unrendered[p] = True
+                    container[key] = value
                     if value.has_inv_query():
                         self._inv_queries.append((p, value))
                         if value.needs_all_envs():
@@ -234,29 +269,34 @@
             if isinstance(value, Value) and value.is_container():
                 value = value.contents()
             if isinstance(value, dict):
-                self._render_simple_dict(value, path.new_subpath(key))
-                container[key] = value
+                container[key] = self._render_simple_dict(value, path.new_subpath(key))
             elif isinstance(value, list):
-                self._render_simple_list(value, path.new_subpath(key))
-                container[key] = value
+                container[key] = self._render_simple_list(value, path.new_subpath(key))
             elif isinstance(value, Value):
                 if value.is_complex():
                     p = path.new_subpath(key)
                     self._unrendered[p] = True
+                    container[key] = value
                     if value.has_inv_query():
                         self._inv_queries.append((p, value))
                         if value.needs_all_envs():
                             self._needs_all_envs = True
                 else:
                     container[key] = value.render(None, None)
+            else:
+                container[key] = value
 
     def _render_simple_dict(self, dictionary, path):
+        new_dict = {}
         for (key, value) in iteritems(dictionary):
-            self._render_simple_container(dictionary, key, value, path)
+            self._render_simple_container(new_dict, key, value, path)
+        return new_dict
 
     def _render_simple_list(self, item_list, path):
+        new_list = [ None ] * len(item_list)
         for n, value in enumerate(item_list):
-            self._render_simple_container(item_list, n, value, path)
+            self._render_simple_container(new_list, n, value, path)
+        return new_list
 
     def interpolate(self, inventory=None):
         self._initialise_interpolate()
@@ -279,7 +319,7 @@
             self._inv_queries = []
             self._needs_all_envs = False
             self._resolve_errors = ResolveErrorList()
-            self._render_simple_dict(self._base, DictPath(self._settings.delimiter))
+            self._base = self._render_simple_dict(self._base, DictPath(self._settings.delimiter))
 
     def _interpolate_inner(self, path, inventory):
         value = path.get_value(self._base)
@@ -305,11 +345,14 @@
                 new = None
             else:
                 raise
+        except InterpolationError as e:
+            e.context = path
+            raise
 
         if isinstance(new, dict):
-            self._render_simple_dict(new, path)
+            new = self._render_simple_dict(new, path)
         elif isinstance(new, list):
-            self._render_simple_list(new, path)
+            new = self._render_simple_list(new, path)
         return new
 
     def _interpolate_references(self, path, value, inventory):
@@ -325,7 +368,7 @@
                         # Therefore, if we encounter False instead of True,
                         # it means that we have already processed it and are now
                         # faced with a cyclical reference.
-                        raise InfiniteRecursionError(path, ref, value.uri())
+                        raise InfiniteRecursionError(path, ref, value.uri)
                     else:
                         self._interpolate_inner(path_from_ref, inventory)
                 else:
@@ -345,4 +388,4 @@
                 old = len(value.get_references())
                 value.assembleRefs(self._base)
                 if old == len(value.get_references()):
-                    raise BadReferencesError(value.get_references(), str(path), value.uri())
+                    raise BadReferencesError(value.get_references(), str(path), value.uri)
diff --git a/reclass/datatypes/tests/test_classes.py b/reclass/datatypes/tests/test_classes.py
index 8d396e0..9b9e419 100644
--- a/reclass/datatypes/tests/test_classes.py
+++ b/reclass/datatypes/tests/test_classes.py
@@ -78,8 +78,9 @@
     def test_append_invalid_characters(self):
         c = Classes()
         invalid_name = ' '.join(('foo', 'bar'))
-        with self.assertRaises(InvalidClassnameError):
+        with self.assertRaises(InvalidClassnameError) as e:
             c.append_if_new(invalid_name)
+        self.assertEqual(e.exception.message, "Invalid character ' ' in class name 'foo bar'.")
 
     def test_merge_unique(self):
         c = Classes(TESTLIST1)
diff --git a/reclass/datatypes/tests/test_entity.py b/reclass/datatypes/tests/test_entity.py
index b09e904..c08a500 100644
--- a/reclass/datatypes/tests/test_entity.py
+++ b/reclass/datatypes/tests/test_entity.py
@@ -253,10 +253,11 @@
         node1_entity.initialise_interpolation()
         node2_entity.initialise_interpolation()
         queries = node1_entity.parameters.get_inv_queries()
-        with self.assertRaises(ResolveError):
+        with self.assertRaises(ResolveError) as e:
             for p, q in queries:
                 node1_entity.interpolate_single_export(q)
                 node2_entity.interpolate_single_export(q)
+        self.assertEqual(e.exception.message, "-> \n   Cannot resolve ${b}, at a")
 
     def test_exports_failed_render_ignore(self):
         node1_exports = Exports({'a': '${a}'}, SETTINGS, '')
diff --git a/reclass/datatypes/tests/test_parameters.py b/reclass/datatypes/tests/test_parameters.py
index fb4a11b..6ab6c93 100644
--- a/reclass/datatypes/tests/test_parameters.py
+++ b/reclass/datatypes/tests/test_parameters.py
@@ -17,7 +17,9 @@
 
 from reclass.settings import Settings
 from reclass.datatypes import Parameters
-from reclass.errors import InfiniteRecursionError, InterpolationError, ResolveError, ResolveErrorList
+from reclass.values.value import Value
+from reclass.values.scaitem import ScaItem
+from reclass.errors import InfiniteRecursionError, InterpolationError, ResolveError, ResolveErrorList, TypeMergeError
 import unittest
 
 try:
@@ -120,13 +122,15 @@
         self.assertEqual(b1.__eq__.call_count, 0)
 
     def test_construct_wrong_type(self):
-        with self.assertRaises(TypeError):
-            self._construct_mocked_params('wrong type')
+        with self.assertRaises(TypeError) as e:
+            self._construct_mocked_params(str('wrong type'))
+        self.assertEqual(e.exception.message, "Cannot merge <type 'str'> objects into Parameters")
 
     def test_merge_wrong_type(self):
         p, b = self._construct_mocked_params()
-        with self.assertRaises(TypeError):
-            p.merge('wrong type')
+        with self.assertRaises(TypeError) as e:
+            p.merge(str('wrong type'))
+        self.assertEqual(e.exception.message, "Cannot merge <type 'str'> objects into Parameters")
 
     def test_get_dict(self):
         p, b = self._construct_mocked_params(SIMPLE)
@@ -138,12 +142,8 @@
         mergee = {'five':5,'four':4,'None':None,'tuple':(1,2,3)}
         p2, b2 = self._construct_mocked_params(mergee)
         p1.merge(p2)
-        p1.initialise_interpolation()
-        for (key, value) in iteritems(mergee):
-            # check that each key, value in mergee resulted in a get call and
-            # a __setitem__ call against b1 (the merge target)
-            self.assertIn(mock.call(key), b1.get.call_args_list)
-            self.assertIn(mock.call(key, value), b1.__setitem__.call_args_list)
+        self.assertEqual(b1.get.call_count, 4)
+        self.assertEqual(b1.__setitem__.call_count, 4)
 
     def test_stray_occurrence_overwrites_during_interpolation(self):
         p1 = Parameters({'r' : mock.sentinel.ref, 'b': '${r}'}, SETTINGS, '')
@@ -183,57 +183,112 @@
         self.assertListEqual(p1.as_dict()['list'], l1+l2)
 
     def test_merge_list_into_scalar(self):
+        l = ['foo', 1, 2]
+        p1 = Parameters(dict(key=l[0]), SETTINGS, '')
+        p2 = Parameters(dict(key=l[1:]), SETTINGS, '')
+        with self.assertRaises(TypeMergeError) as e:
+            p1.merge(p2)
+            p1.interpolate()
+        self.assertEqual(e.exception.message, "-> \n   Canot merge list over scalar, at key, in ; ")
+
+    def test_merge_list_into_scalar_allow(self):
         settings = Settings({'allow_list_over_scalar': True})
         l = ['foo', 1, 2]
         p1 = Parameters(dict(key=l[0]), settings, '')
         p2 = Parameters(dict(key=l[1:]), settings, '')
         p1.merge(p2)
-        p1.initialise_interpolation()
+        p1.interpolate()
         self.assertListEqual(p1.as_dict()['key'], l)
 
     def test_merge_scalar_over_list(self):
         l = ['foo', 1, 2]
+        p1 = Parameters(dict(key=l[:2]), SETTINGS, '')
+        p2 = Parameters(dict(key=l[2]), SETTINGS, '')
+        with self.assertRaises(TypeMergeError) as e:
+            p1.merge(p2)
+            p1.interpolate()
+        self.assertEqual(e.exception.message, "-> \n   Canot merge scalar over list, at key, in ; ")
+
+    def test_merge_scalar_over_list_allow(self):
+        l = ['foo', 1, 2]
         settings = Settings({'allow_scalar_over_list': True})
         p1 = Parameters(dict(key=l[:2]), settings, '')
         p2 = Parameters(dict(key=l[2]), settings, '')
         p1.merge(p2)
-        p1.initialise_interpolation()
+        p1.interpolate()
         self.assertEqual(p1.as_dict()['key'], l[2])
 
     def test_merge_none_over_list(self):
         l = ['foo', 1, 2]
+        settings = Settings({'allow_none_override': False})
+        p1 = Parameters(dict(key=l[:2]), settings, '')
+        p2 = Parameters(dict(key=None), settings, '')
+        with self.assertRaises(TypeMergeError) as e:
+            p1.merge(p2)
+            p1.interpolate()
+        self.assertEqual(e.exception.message, "-> \n   Canot merge scalar over list, at key, in ; ")
+
+    def test_merge_none_over_list_allow(self):
+        l = ['foo', 1, 2]
         settings = Settings({'allow_none_override': True})
         p1 = Parameters(dict(key=l[:2]), settings, '')
         p2 = Parameters(dict(key=None), settings, '')
         p1.merge(p2)
-        p1.initialise_interpolation()
+        p1.interpolate()
         self.assertEqual(p1.as_dict()['key'], None)
 
-    def test_merge_none_over_list_negative(self):
-        l = ['foo', 1, 2]
-        settings = Settings({'allow_none_override': False})
-        p1 = Parameters(dict(key=l[:2]), settings, '')
-        p2 = Parameters(dict(key=None), settings, '')
-        with self.assertRaises(TypeError):
+    def test_merge_dict_over_scalar(self):
+        d = { 'one': 1, 'two': 2 }
+        p1 = Parameters({ 'a': 1 }, SETTINGS, '')
+        p2 = Parameters({ 'a': d }, SETTINGS, '')
+        with self.assertRaises(TypeMergeError) as e:
             p1.merge(p2)
-            p1.initialise_interpolation()
+            p1.interpolate()
+        self.assertEqual(e.exception.message, "-> \n   Canot merge dictionary over scalar, at a, in ; ")
+
+    def test_merge_dict_over_scalar_allow(self):
+        settings = Settings({'allow_dict_over_scalar': True})
+        d = { 'one': 1, 'two': 2 }
+        p1 = Parameters({ 'a': 1 }, settings, '')
+        p2 = Parameters({ 'a': d }, settings, '')
+        p1.merge(p2)
+        p1.interpolate()
+        self.assertEqual(p1.as_dict(), { 'a': d })
+
+    def test_merge_scalar_over_dict(self):
+        d = { 'one': 1, 'two': 2}
+        p1 = Parameters({ 'a': d }, SETTINGS, '')
+        p2 = Parameters({ 'a': 1 }, SETTINGS, '')
+        with self.assertRaises(TypeMergeError) as e:
+            p1.merge(p2)
+            p1.interpolate()
+        self.assertEqual(e.exception.message, "-> \n   Canot merge scalar over dictionary, at a, in ; ")
+
+    def test_merge_scalar_over_dict_allow(self):
+        d = { 'one': 1, 'two': 2}
+        settings = Settings({'allow_scalar_over_dict': True})
+        p1 = Parameters({ 'a': d }, settings, '')
+        p2 = Parameters({ 'a': 1 }, settings, '')
+        p1.merge(p2)
+        p1.interpolate()
+        self.assertEqual(p1.as_dict(), { 'a': 1})
 
     def test_merge_none_over_dict(self):
+        p1 = Parameters(dict(key=SIMPLE), SETTINGS, '')
+        p2 = Parameters(dict(key=None), SETTINGS, '')
+        with self.assertRaises(TypeMergeError) as e:
+            p1.merge(p2)
+            p1.interpolate()
+        self.assertEqual(e.exception.message, "-> \n   Canot merge scalar over dictionary, at key, in ; ")
+
+    def test_merge_none_over_dict_allow(self):
         settings = Settings({'allow_none_override': True})
         p1 = Parameters(dict(key=SIMPLE), settings, '')
         p2 = Parameters(dict(key=None), settings, '')
         p1.merge(p2)
-        p1.initialise_interpolation()
+        p1.interpolate()
         self.assertEqual(p1.as_dict()['key'], None)
 
-    def test_merge_none_over_dict_negative(self):
-        settings = Settings({'allow_none_override': False})
-        p1 = Parameters(dict(key=SIMPLE), settings, '')
-        p2 = Parameters(dict(key=None), settings, '')
-        with self.assertRaises(TypeError):
-            p1.merge(p2)
-            p1.initialise_interpolation()
-
     # def test_merge_bare_dict_over_dict(self):
         # settings = Settings({'allow_bare_override': True})
         # p1 = Parameters(dict(key=SIMPLE), settings, '')
@@ -285,22 +340,6 @@
         p.initialise_interpolation()
         self.assertDictEqual(p.as_dict(), dict(dict=goal))
 
-    def test_merge_dict_into_scalar(self):
-        p = Parameters(dict(base='foo'), SETTINGS, '')
-        p2 = Parameters(dict(base=SIMPLE), SETTINGS, '')
-        with self.assertRaises(TypeError):
-            p.merge(p2)
-            p.interpolate()
-
-    def test_merge_scalar_over_dict(self):
-        settings = Settings({'allow_scalar_over_dict': True})
-        p = Parameters(dict(base=SIMPLE), settings, '')
-        mergee = {'base':'foo'}
-        p2 = Parameters(mergee, settings, '')
-        p.merge(p2)
-        p.initialise_interpolation()
-        self.assertDictEqual(p.as_dict(), mergee)
-
     def test_interpolate_single(self):
         v = 42
         d = {'foo': 'bar'.join(SETTINGS.reference_sentinels),
@@ -340,8 +379,9 @@
         d = {'foo': 'bar'.join(SETTINGS.reference_sentinels),
              'bar': 'foo'.join(SETTINGS.reference_sentinels)}
         p = Parameters(d, SETTINGS, '')
-        with self.assertRaises(InfiniteRecursionError):
+        with self.assertRaises(InfiniteRecursionError) as e:
             p.interpolate()
+        self.assertEqual(e.exception.message, "-> \n   Infinite recursion: ${foo}, at bar")
 
     def test_nested_references(self):
         d = {'a': '${${z}}', 'b': 2, 'z': 'b'}
@@ -423,6 +463,22 @@
         p1.interpolate()
         self.assertEqual(p1.as_dict(), r)
 
+    def test_overwrite_dict(self):
+        p1 = Parameters({'a': { 'one': 1, 'two': 2 }}, SETTINGS, '')
+        p2 = Parameters({'~a': { 'three': 3, 'four': 4 }}, SETTINGS, '')
+        r = {'a': { 'three': 3, 'four': 4 }}
+        p1.merge(p2)
+        p1.interpolate()
+        self.assertEqual(p1.as_dict(), r)
+
+    def test_overwrite_list(self):
+        p1 = Parameters({'a': [1, 2]}, SETTINGS, '')
+        p2 = Parameters({'~a': [3, 4]}, SETTINGS, '')
+        r = {'a': [3, 4]}
+        p1.merge(p2)
+        p1.interpolate()
+        self.assertEqual(p1.as_dict(), r)
+
     def test_interpolate_escaping(self):
         v = 'bar'.join(SETTINGS.reference_sentinels)
         d = {'foo': SETTINGS.escape_character + 'bar'.join(SETTINGS.reference_sentinels),
diff --git a/reclass/errors.py b/reclass/errors.py
index 800a2f8..fdd7f38 100644
--- a/reclass/errors.py
+++ b/reclass/errors.py
@@ -208,6 +208,7 @@
         msg = 'Cannot resolve {0}'.format(self.reference.join(REFERENCE_SENTINELS)) + self._add_context_and_uri()
         return [ msg ]
 
+
 class ResolveErrorList(InterpolationError):
     def __init__(self):
         super(ResolveErrorList, self).__init__(msg=None)
@@ -280,6 +281,18 @@
         return msg
 
 
+class TypeMergeError(InterpolationError):
+
+    def __init__(self, value1, value2, uri):
+        super(TypeMergeError, self).__init__(msg=None, uri=uri, tbFlag=False)
+        self.type1 = value1.item_type_str()
+        self.type2 = value2.item_type_str()
+
+    def _get_error_message(self):
+        msg = [ 'Canot merge {0} over {1}'.format(self.type1, self.type2) + self._add_context_and_uri() ]
+        return msg
+
+
 class ExpressionError(InterpolationError):
 
     def __init__(self, msg, rc=posix.EX_DATAERR, tbFlag=True):
diff --git a/reclass/values/compitem.py b/reclass/values/compitem.py
index 7928e6f..183bc43 100644
--- a/reclass/values/compitem.py
+++ b/reclass/values/compitem.py
@@ -46,17 +46,7 @@
     def merge_over(self, item):
         if item.type == Item.SCALAR or item.type == Item.COMPOSITE:
             return self
-        elif item.type == Item.LIST:
-            if self._settings.allow_scalar_over_list or (self._settings.allow_none_override and self._value in [None, 'none', 'None']):
-                return self
-            else:
-                raise TypeError('allow scalar over list = False: cannot merge %s over %s' % (repr(self), repr(item)))
-        elif item.type == Item.DICTIONARY:
-            if self._settings.allow_scalar_over_dict or (self._settings.allow_none_override and self._value in [None, 'none', 'None']):
-                return self
-            else:
-                raise TypeError('allow scalar over dict = False: cannot merge %s over %s' % (repr(self), repr(item)))
-        raise TypeError('Cannot merge %s over %s' % (repr(self), repr(item)))
+        raise RuntimeError('Trying to merge %s over %s' % (repr(self), repr(item)))
 
     def render(self, context, inventory):
         # Preserve type if only one item
diff --git a/reclass/values/dictitem.py b/reclass/values/dictitem.py
index 76cefe2..d5272b9 100644
--- a/reclass/values/dictitem.py
+++ b/reclass/values/dictitem.py
@@ -24,14 +24,6 @@
     def is_container(self):
         return True
 
-    def merge_over(self, item):
-        if item.type == Item.SCALAR:
-            if item.contents() is None or self._settings.allow_dict_over_scalar:
-                return self
-            else:
-                raise TypeError('allow dict over scalar = False: cannot merge %s onto %s' % (repr(self), repr(item)))
-        raise TypeError('Cannot merge %s over %s' % (repr(self), repr(item)))
-
     def render(self, context, inventory):
         return self._dict
 
diff --git a/reclass/values/item.py b/reclass/values/item.py
index bc507f4..cad3684 100644
--- a/reclass/values/item.py
+++ b/reclass/values/item.py
@@ -19,6 +19,10 @@
     REFERENCE = 5
     SCALAR = 6
 
+    TYPE_STR = { COMPOSITE: 'composite', DICTIONARY: 'dictionary',
+                 INV_QUERY: 'invventory query', LIST: 'list',
+                 REFERENCE: 'reference', SCALAR: 'scalar' }
+
     def allRefs(self):
         return True
 
@@ -45,3 +49,6 @@
     def render(self, context, exports):
         msg = "Item class {0} does not implement render()"
         raise NotImplementedError(msg.format(self.__class__.__name__))
+
+    def type_str(self):
+        return self.TYPE_STR[self.type]
diff --git a/reclass/values/listitem.py b/reclass/values/listitem.py
index 8f1a21d..41c02dd 100644
--- a/reclass/values/listitem.py
+++ b/reclass/values/listitem.py
@@ -31,15 +31,7 @@
         if item.type == Item.LIST:
             item._list.extend(self._list)
             return item
-        elif item.type == Item.SCALAR:
-            if item.contents() is None:
-                return self
-            elif self._settings.allow_list_over_scalar:
-                self._list.insert(0, item.contents())
-                return self
-            else:
-                raise TypeError('allow list over scalar = False: cannot merge %s onto %s' % (repr(self), repr(item)))
-        raise TypeError('Cannot merge %s over %s' % (repr(self), repr(item)))
+        raise RuntimeError('Trying to merge %s over %s' % (repr(self), repr(item)))
 
     def __repr__(self):
         return 'ListItem(%r)' % (self._list)
diff --git a/reclass/values/scaitem.py b/reclass/values/scaitem.py
index f3057cb..c16ab45 100644
--- a/reclass/values/scaitem.py
+++ b/reclass/values/scaitem.py
@@ -24,17 +24,7 @@
     def merge_over(self, item):
         if item.type == Item.SCALAR or item.type == Item.COMPOSITE:
             return self
-        elif item.type == Item.LIST:
-            if self._settings.allow_scalar_over_list or (self._settings.allow_none_override and self._value is None):
-                return self
-            else:
-                raise TypeError('allow scalar over list = False: cannot merge %s over %s' % (repr(self), repr(item)))
-        elif item.type == Item.DICTIONARY:
-            if self._settings.allow_scalar_over_dict or (self._settings.allow_none_override and self._value is None):
-                return self
-            else:
-                raise TypeError('allow scalar over dict = False: cannot merge %s over %s' % (repr(self), repr(item)))
-        raise TypeError('Cannot merge %s over %s' % (repr(self), repr(item)))
+        raise RuntimeError('Trying to merge %s over %s' % (repr(self), repr(item)))
 
     def render(self, context, inventory):
         return self._value
diff --git a/reclass/values/value.py b/reclass/values/value.py
index f87a9b4..286407c 100644
--- a/reclass/values/value.py
+++ b/reclass/values/value.py
@@ -48,9 +48,16 @@
     def overwrite(self, overwrite):
         self._overwrite = overwrite
 
+    @property
     def uri(self):
         return self._uri
 
+    def item_type(self):
+        return self._item.type
+
+    def item_type_str(self):
+        return self._item.type_str()
+
     def is_container(self):
         return self._item.is_container()
 
diff --git a/reclass/values/valuelist.py b/reclass/values/valuelist.py
index 8b8dd51..adffb87 100644
--- a/reclass/values/valuelist.py
+++ b/reclass/values/valuelist.py
@@ -13,7 +13,7 @@
 import copy
 import sys
 
-from reclass.errors import ResolveError
+from reclass.errors import ResolveError, TypeMergeError
 
 class ValueList(object):
 
@@ -25,8 +25,13 @@
         self._inv_refs = []
         self._has_inv_query = False
         self._ignore_failed_render = False
+        self._is_complex = False
         self._update()
 
+    @property
+    def uri(self):
+        return '; '.join([ str(x.uri) for x in self._values ])
+
     def append(self, value):
         self._values.append(value)
         self._update()
@@ -38,9 +43,11 @@
     def _update(self):
         self.assembleRefs()
         self._check_for_inv_query()
-
-    def uri(self):
-        return '; '.join([ x.uri() for x in self._values ])
+        self._is_complex = False
+        item_type = self._values[0].item_type()
+        for v in self._values:
+            if v.is_complex() or v.overwrite or v.item_type() != item_type:
+                self._is_complex = True
 
     def has_references(self):
         return len(self._refs) > 0
@@ -52,7 +59,7 @@
         return self._inv_refs
 
     def is_complex(self):
-        return (self.has_references() | self.has_inv_query())
+        return self._is_complex
 
     def get_references(self):
         return self._refs
@@ -104,6 +111,8 @@
             try:
                 new = value.render(context, inventory)
             except ResolveError as e:
+                # only ignore failed renders if ignore_overwritten_missing_references is set and we are dealing with a scalar value
+                # and it's not the last item in the values list
                 if self._settings.ignore_overwritten_missing_references and not isinstance(output, (dict, list)) and n != (len(self._values)-1):
                     new = None
                     last_error = e
@@ -115,23 +124,51 @@
                 output = new
                 deepCopied = False
             else:
-                if isinstance(output, dict) and isinstance(new, dict):
-                    p1 = Parameters(output, self._settings, None, parse_strings=False)
-                    p2 = Parameters(new, self._settings, None, parse_strings=False)
-                    p1.merge(p2)
-                    output = p1.as_dict()
-                    continue
-                elif isinstance(output, list) and isinstance(new, list):
-                    if not deepCopied:
-                        output = copy.deepcopy(output)
-                        deepCopied = True
-                    output.extend(new)
-                    continue
-                elif isinstance(output, (dict, list)) or isinstance(new, (dict, list)):
-                    raise TypeError('Cannot merge %s over %s' % (repr(self._values[n]), repr(self._values[n-1])))
+                if isinstance(output, dict):
+                    if isinstance(new, dict):
+                        p1 = Parameters(output, self._settings, None, parse_strings=False)
+                        p2 = Parameters(new, self._settings, None, parse_strings=False)
+                        p1.merge(p2)
+                        output = p1.as_dict()
+                    elif isinstance(new, list):
+                        raise TypeMergeError(self._values[n], self._values[n-1], self.uri)
+                    elif self._settings.allow_scalar_over_dict or (self._settings.allow_none_override and new is None):
+                        output = new
+                        deepCopied = False
+                    else:
+                        raise TypeMergeError(self._values[n], self._values[n-1], self.uri)
+                elif isinstance(output, list):
+                    if isinstance(new, list):
+                        if not deepCopied:
+                            output = copy.deepcopy(output)
+                            deepCopied = True
+                        output.extend(new)
+                    elif isinstance(new, dict):
+                        raise TypeMergeError(self._values[n], self._values[n-1], self.uri)
+                    elif self._settings.allow_scalar_over_list or (self._settings.allow_none_override and new is None):
+                        output = new
+                        deepCopied = False
+                    else:
+                        raise TypeMergeError(self._values[n], self._values[n-1], self.uri)
                 else:
-                    output = new
-                    deepCopied = False
+                    if isinstance(new, dict):
+                        if self._settings.allow_dict_over_scalar:
+                            output = new
+                            deepCopied = False
+                        else:
+                            raise TypeMergeError(self._values[n], self._values[n-1], self.uri)
+                    elif isinstance(new, list):
+                        if self._settings.allow_list_over_scalar:
+                            output_list = list()
+                            output_list.append(output)
+                            output_list.extend(new)
+                            output = output_list
+                            deepCopied = True
+                        else:
+                            raise TypeMergeError(self._values[n], self._values[n-1], self.uri)
+                    else:
+                        output = new
+                        deepCopied = False
 
         if isinstance(output, (dict, list)) and last_error is not None:
             raise last_error