diff -r 5ebc4a192550 -r b481bf14992d mercurial/revlog.py
--- a/mercurial/revlog.py	Tue Dec 01 15:29:11 2015 -0800
+++ b/mercurial/revlog.py	Tue Dec 01 16:22:49 2015 -0800
@@ -1459,6 +1459,7 @@
                         delta = builddelta(prev)
             else:
                 delta = builddelta(prev)
+        if delta is not None:
             dist, l, data, base, chainbase, chainlen, compresseddeltalen = delta
 
         if not self._isgooddelta(delta, textlen):