Ticket #3851: ticket3851.diff

django/db/models/manipulators.py

@@ -295 +295 @@
             # doesn't have blank=True.
             return
         if isinstance(f.rel, ManyToOneRel):
-            kwargs['%s__pk' % f.name] = field_val
+            if field_val == '':
+                kwargs['%s__isnull' % f.name] = True
+            else:
+                kwargs['%s__pk' % f.name] = field_val
         else:
             kwargs['%s__iexact' % f.name] = field_val
     try: