Index: django/newforms/models.py
===================================================================
--- django/newforms/models.py	(Revision 7189)
+++ django/newforms/models.py	(Arbeitskopie)
@@ -250,6 +250,8 @@
             self.instance = opts.model()
             object_data = {}
         else:
+            assert isinstance(instance, opts.model), 'instance (%r)  must be an instance of %r' % (
+                instance, opts.model)
             self.instance = instance
             object_data = model_to_dict(instance, opts.fields, opts.exclude)
         # if initial was provided, it should override the values from instance