mercurial/discovery.py

--- a/mercurial/discovery.py	Mon May 01 15:40:41 2017 +0200
+++ b/mercurial/discovery.py	Wed Apr 19 23:10:05 2017 +0900
@@ -511,7 +511,7 @@
         for m in markers:
             nexts = m[1] # successors
             if not nexts: # this is a prune marker
-                nexts = m[5] # parents
+                nexts = m[5] or () # parents
             for n in nexts:
                 if n not in seen:
                     seen.add(n)