check if compostite_class is of class type

before checking if it is a subclass of `MutableComposite`

1 files changed, 2 insertions, 1 deletions

diff --git a/lib/sqlalchemy/ext/mutable.py b/lib/sqlalchemy/ext/mutable.py
index ca27f172f..f503ad6d4 100644
--- a/lib/sqlalchemy/ext/mutable.py
+++ b/lib/sqlalchemy/ext/mutable.py
@@ -588,9 +588,10 @@ class MutableComposite(MutableBase):
                 setattr(parent, attr_name, value)
 
 def _setup_composite_listener():
+    import types
     def _listen_for_type(mapper, class_):
         for prop in mapper.iterate_properties:
-            if (hasattr(prop, 'composite_class') and
+            if (hasattr(prop, 'composite_class') and (type(prop.composite_class) is types.ClassType) and
                 issubclass(prop.composite_class, MutableComposite)):
                 prop.composite_class._listen_on_attribute(
                     getattr(class_, prop.key), False, class_)