diff --git a/mercurial/copies.py b/mercurial/copies.py
--- a/mercurial/copies.py
+++ b/mercurial/copies.py
@@ -370,6 +370,7 @@ def mergecopies(repo, c1, c2, ca):
 
     # generate a directory move map
     d1, d2 = c1.dirs(), c2.dirs()
+    # Hack for adding '', which is not otherwise added, to d1 and d2
     d1.addpath('/')
     d2.addpath('/')
     invalid = set()