diff --git a/reclass/datatypes/classes.py b/reclass/datatypes/classes.py
index 132db31..2591bc1 100644
--- a/reclass/datatypes/classes.py
+++ b/reclass/datatypes/classes.py
@@ -8,9 +8,10 @@
 #
 
 import types
+import os
 from reclass.errors import InvalidClassnameError
 
-INVALID_CHARACTERS_FOR_CLASSNAMES = ' '
+INVALID_CHARACTERS_FOR_CLASSNAMES = ' ' + os.sep
 
 class Classes(object):
     '''
@@ -57,8 +58,7 @@
     def _assert_valid_characters(self, item):
         for c in INVALID_CHARACTERS_FOR_CLASSNAMES:
             if c in item:
-                raise InvalidClassnameError("Invalid character '{0}' "
-                                            "in class name '{1}'.".format(c, item))
+                raise InvalidClassnameError(c, item)
 
     def _append_if_new(self, item):
         if item not in self._items:
diff --git a/reclass/errors.py b/reclass/errors.py
index b7940d6..53f340a 100644
--- a/reclass/errors.py
+++ b/reclass/errors.py
@@ -111,5 +111,9 @@
 
 class InvalidClassnameError(NameError):
 
-    def __init__(self, msg):
+    def __init__(self, invalid_character, classname):
+        self._invalid_character = invalid_character
+        self._classname = classname
+        msg = "Invalid character '{0}' in class name '{1}'."
+        msg = msg.format(invalid_character, classname)
         super(InvalidClassnameError, self).__init__(msg)