--- a/mercurial/revlog.py	Fri Dec 04 17:22:26 2015 -0800
+++ b/mercurial/revlog.py	Fri Dec 04 16:45:06 2015 -0800
@@ -1428,7 +1428,7 @@
                 # Assume what we received from the server is a good choice
                 # build delta will reuse the cache
                 candidatedelta = builddelta(cachedelta[0])
-                tested.add(candidatedelta[3])
+                tested.add(cachedelta[0])
                 if self._isgooddelta(candidatedelta, textlen):
                     delta = candidatedelta
             if delta is None and self._generaldelta: