diff -r cab19d49f8bd -r 02fa567f8a3c mercurial/merge.py
--- a/mercurial/merge.py	Sun Mar 31 09:39:02 2019 -0700
+++ b/mercurial/merge.py	Sat Mar 30 13:13:10 2019 -0700
@@ -2205,7 +2205,7 @@
                   error=stats.unresolvedcount)
     return stats
 
-def graft(repo, ctx, pctx, labels, keepparent=False,
+def graft(repo, ctx, pctx, labels=None, keepparent=False,
           keepconflictparent=False):
     """Do a graft-like merge.