diff --git a/src/calibre/db/tables.py b/src/calibre/db/tables.py
index edb16d8e3a..00eeaa5a1d 100644
--- a/src/calibre/db/tables.py
+++ b/src/calibre/db/tables.py
@@ -41,9 +41,7 @@ def c_parse(val):
 
 ONE_ONE, MANY_ONE, MANY_MANY = xrange(3)
 
-class Null:
-    pass
-null = Null()
+null = object()
 
 class Table(object):