--- a/mercurial/copies.py	Mon Jun 25 15:14:05 2012 -0700
+++ b/mercurial/copies.py	Wed Jun 27 13:41:04 2012 -0500
@@ -321,7 +321,9 @@
 
     # generate a directory move map
     d1, d2 = c1.dirs(), c2.dirs()
-    invalid = set([""])
+    d1.add('')
+    d2.add('')
+    invalid = set()
     dirmove = {}
 
     # examine each file copy for a potential directory move, which is