Index: db/models/fields/__init__.py
===================================================================
--- db/models/fields/__init__.py	(revision 9084)
+++ db/models/fields/__init__.py	(working copy)
@@ -350,6 +350,10 @@
     def get_db_prep_value(self, value):
         if value is None:
             return None
+        try:
+            int(value)
+        except:
+            return None
         return int(value)
 
     def contribute_to_class(self, cls, name):