diff -r f78eadbb5769 -r f9a80054dd3c mercurial/context.py
--- a/mercurial/context.py	Wed May 20 00:43:23 2009 +0200
+++ b/mercurial/context.py	Wed May 20 00:52:46 2009 +0200
@@ -444,7 +444,7 @@
 
         # prime the ancestor cache for the working directory
         for c in (self, fc2):
-            if c._filerev == None:
+            if c._filerev is None:
                 pl = [(n.path(), n.filenode()) for n in c.parents()]
                 acache[(c._path, None)] = pl