diff -r d61c42c1a35c -r cdef35b38026 mercurial/revset.py
--- a/mercurial/revset.py	Mon Oct 10 22:30:09 2016 +0200
+++ b/mercurial/revset.py	Fri Oct 14 23:33:00 2016 +0900
@@ -1621,7 +1621,7 @@
             ps.add(cl.parentrevs(r)[0])
         elif n == 2:
             parents = cl.parentrevs(r)
-            if len(parents) > 1:
+            if parents[1] != node.nullrev:
                 ps.add(parents[1])
     return subset & ps