diff -r 576eef1ab43d -r 7e75777e4a51 mercurial/configitems.py
--- a/mercurial/configitems.py	Thu Aug 02 14:57:20 2018 -0700
+++ b/mercurial/configitems.py	Sun Aug 05 00:53:55 2018 -0700
@@ -194,7 +194,7 @@
     default=False,
 )
 coreconfigitem('commands', 'resolve.mark-check',
-    default=None,
+    default='none',
 )
 coreconfigitem('commands', 'show.aliasprefix',
     default=list,