diff --git a/mergers/dict/recursive_policy_update.py b/mergers/dict/recursive_policy_update.py
index 01dfccb..e280889 100644
--- a/mergers/dict/recursive_policy_update.py
+++ b/mergers/dict/recursive_policy_update.py
@@ -13,10 +13,15 @@
     def __init__(self, policy=None):
         super(DictRecursivePolicyUpdate, self).__init__()
         if policy is None:
-            policy = {(dict,dict) : self.merge,
-                      (list,list) : lambda x,y: x+y,
-                      (dict,list) : lambda x,y: self.merge(x, dict(y)),
-                      None        : lambda x,y: y
+            first = lambda first, second: first
+            second = lambda first, second: second
+
+            policy = {(dict,dict)       : self.merge,
+                      (list,list)       : lambda x,y: x+y,
+                      (dict,list)       : lambda x,y: self.merge(x, dict(y)),
+                      (dict,type(None)) : first,
+                      (list,type(None)) : first,
+                      None              : second
                      }
         self._policy = policy
 
@@ -27,8 +32,8 @@
         ret = first.copy()
         for k,v in second.iteritems():
             if k in ret:
-                pfn = self._policy.get((type(ret[k]), type(v)),
-                                       self._policy.get(None))
+                lookup = (type(ret[k]), type(v))
+                pfn = self._policy.get(lookup, self._policy.get(None))
                 ret[k] = pfn(ret[k], v)
             else:
                 ret[k] = v
diff --git a/storage/yaml_fs/tests/classes/debiannode.yml b/storage/yaml_fs/tests/classes/debiannode.yml
index 43e35e4..84837ff 100644
--- a/storage/yaml_fs/tests/classes/debiannode.yml
+++ b/storage/yaml_fs/tests/classes/debiannode.yml
@@ -2,3 +2,6 @@
 - basenode
 applications:
 - apt
+parameters:
+  apt:
+    mirror_base: http://http.debian.net
diff --git a/storage/yaml_fs/tests/nodes/red.yml b/storage/yaml_fs/tests/nodes/red.yml
index 5fe618d..051d0f5 100644
--- a/storage/yaml_fs/tests/nodes/red.yml
+++ b/storage/yaml_fs/tests/nodes/red.yml
@@ -3,6 +3,7 @@
 - hosted@zurich
 - webserver
 parameters:
+  apt:  # null-value must not override inherited dictionary
   motd:
     greeting: This node is $nodename
   colour: red
diff --git a/storage/yaml_fs/tests/test_yaml_fs.py b/storage/yaml_fs/tests/test_yaml_fs.py
index bb4ba12..89ae294 100644
--- a/storage/yaml_fs/tests/test_yaml_fs.py
+++ b/storage/yaml_fs/tests/test_yaml_fs.py
@@ -63,3 +63,7 @@
             assert 'applications' in node
             assert 'classes' in node
             assert 'parameters' in node
+
+    def test_merge_empty_dict(self):
+        node = self._storage.nodeinfo(HOSTS[0])
+        assert node['parameters'].get('apt') is not None