--- a/mercurial/manifest.py	Fri Sep 29 14:49:05 2017 -0700
+++ b/mercurial/manifest.py	Sat Sep 30 05:22:22 2017 +0530
@@ -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):