diff -r 576eef1ab43d -r 7e75777e4a51 mercurial/commands.py
--- a/mercurial/commands.py	Thu Aug 02 14:57:20 2018 -0700
+++ b/mercurial/commands.py	Sun Aug 05 00:53:55 2018 -0700
@@ -4608,6 +4608,9 @@
         hasconflictmarkers = []
         if mark:
             markcheck = ui.config('commands', 'resolve.mark-check')
+            if markcheck not in ['warn', 'abort']:
+                # Treat all invalid / unrecognized values as 'none'.
+                markcheck = False
         for f in ms:
             if not m(f):
                 continue