diff -r b913c394386f -r 2a038deeac9a mercurial/merge.py
--- a/mercurial/merge.py	Thu Nov 27 12:25:01 2014 -0600
+++ b/mercurial/merge.py	Sun Nov 30 19:26:53 2014 +0100
@@ -968,7 +968,7 @@
         pl = wc.parents()
         p1 = pl[0]
         pas = [None]
-        if ancestor:
+        if ancestor is not None:
             pas = [repo[ancestor]]
 
         if node is None: