diff -r ac666c5c2e0c -r a6014392837e mercurial/context.py
--- a/mercurial/context.py	Wed Apr 04 15:11:43 2018 -0700
+++ b/mercurial/context.py	Tue Apr 03 23:47:01 2018 -0700
@@ -631,7 +631,7 @@
             # experimental config: merge.preferancestor
             for r in self._repo.ui.configlist('merge', 'preferancestor'):
                 try:
-                    ctx = changectx(self._repo, r)
+                    ctx = scmutil.revsymbol(self._repo, r)
                 except error.RepoLookupError:
                     continue
                 anc = ctx.node()