diff -r ef303daefdf7 -r 99c3dee3f6ce mercurial/context.py
--- a/mercurial/context.py	Fri Jun 30 03:39:10 2017 +0200
+++ b/mercurial/context.py	Fri Jun 30 03:43:13 2017 +0200
@@ -659,7 +659,7 @@
             anc = cahs[0]
         else:
             # experimental config: merge.preferancestor
-            for r in self._repo.ui.configlist('merge', 'preferancestor', ['*']):
+            for r in self._repo.ui.configlist('merge', 'preferancestor'):
                 try:
                     ctx = changectx(self._repo, r)
                 except error.RepoLookupError: