--- a/mercurial/manifest.py	Fri Dec 12 15:29:54 2014 -0500
+++ b/mercurial/manifest.py	Fri Dec 12 13:40:44 2014 -0500
@@ -17,6 +17,9 @@
             flags = {}
         dict.__init__(self, mapping)
         self._flags = flags
+    def __setitem__(self, k, v):
+        assert v is not None
+        dict.__setitem__(self, k, v)
     def flags(self, f):
         return self._flags.get(f, "")
     def withflags(self):