Use a more explicit check for whether these ids are None.

        if obj is not None:

            return ContentType.objects.db_manager(obj._state.db).get_for_model(

                obj, for_concrete_model=self.for_concrete_model)

        elif id:

        elif id is not None:

            return ContentType.objects.db_manager(using).get_for_id(id)

        else:

            # This should never happen. I love comments like this, don't you?

            # performance when dealing with GFKs in loops and such.

            f = self.model._meta.get_field(self.ct_field)

            ct_id = getattr(instance, f.get_attname(), None)

            if ct_id:

            if ct_id is not None:

                ct = self.get_content_type(id=ct_id, using=instance._state.db)

                try:

                    rel_obj = ct.get_object_for_this_type(pk=getattr(instance, self.fk_field))