diff --git a/mercurial/manifest.py b/mercurial/manifest.py
--- a/mercurial/manifest.py
+++ b/mercurial/manifest.py
@@ -442,6 +442,8 @@
         self._lm[key] = node, self.flags(key, '')
 
     def __contains__(self, key):
+        if key is None:
+            return False
         return key in self._lm
 
     def __delitem__(self, key):