diff -r 9903261dcc81 -r 652dfb1eff14 mercurial/commands.py
--- a/mercurial/commands.py	Thu Oct 15 01:11:00 2015 +0100
+++ b/mercurial/commands.py	Thu Oct 15 01:19:32 2015 +0100
@@ -4901,7 +4901,7 @@
         node = scmutil.revsingle(repo, node).node()
 
     if not node:
-        node = scmutil.revsingle(repo, '_mergedefaultdest()').node()
+        node = repo[destutil.destmerge(repo)].node()
 
     if opts.get('preview'):
         # find nodes that are ancestors of p2 but not of p1