Index: query.py
===================================================================
--- query.py	(revision 3763)
+++ query.py	(working copy)
@@ -707,6 +707,9 @@
     joins, where, params = SortedDict(), [], []
 
     for kwarg, value in kwarg_items:
+        if value is None:
+            value = True
+            kwark = kwarg + "__isnull"    
         if value is not None:
             path = kwarg.split(LOOKUP_SEPARATOR)
             # Extract the last elements of the kwarg.