--- a/mercurial/merge.py	Wed May 31 23:42:58 2017 +0530
+++ b/mercurial/merge.py	Wed May 31 23:48:52 2017 +0530
@@ -801,7 +801,7 @@
 
     # manifests fetched in order are going to be faster, so prime the caches
     [x.manifest() for x in
-     sorted(wctx.parents() + [p2, pa], key=lambda x: x.rev())]
+     sorted(wctx.parents() + [p2, pa], key=lambda x: x.rev() or -1)]
 
     if followcopies:
         ret = copies.mergecopies(repo, wctx, p2, pa)