Fix typo.

src/mongopersist/serialize.py

@@ -145,7 +145,7 @@ def get_non_persistent_state(self, obj, seen):
         if id(obj) in seen:
             raise interfaces.CircularReferenceError(obj)
         # Add the current object to the list of seen objects.
-        if not (type(obj) in interfaces.REFERENCE_SAFE_TYPES) or
+        if not (type(obj) in interfaces.REFERENCE_SAFE_TYPES or
                 getattr(obj, '_m_reference_safe', False)):
             seen.append(id(obj))
         # Get the state of the object. Only pickable objects can be reduced.