diff --git a/mercurial/manifest.py b/mercurial/manifest.py
--- a/mercurial/manifest.py
+++ b/mercurial/manifest.py
@@ -960,7 +960,7 @@
                 else:
                     files.update(m1.iterkeys())
 
-            for fn in t1._files.iterkeys():
+            for fn in t1._files:
                 if fn not in t2._files:
                     files.add(t1._subpath(fn))