diff -r 44be33cf7a57 -r 2c14bf83b58b mercurial/merge.py
--- a/mercurial/merge.py	Tue Sep 17 15:35:16 2019 -0700
+++ b/mercurial/merge.py	Thu Sep 19 07:50:24 2019 +0900
@@ -2257,7 +2257,7 @@
         pother = nullid
         parents = ctx.parents()
         if keepparent and len(parents) == 2 and pctx in parents:
-            if pctx == parents[0]:
+            if pctx == parents[1]:
                 potherp1 = True
             parents.remove(pctx)
             pother = parents[0].node()