diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -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
diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -194,7 +194,7 @@
     default=False,
 )
 coreconfigitem('commands', 'resolve.mark-check',
-    default=None,
+    default='none',
 )
 coreconfigitem('commands', 'show.aliasprefix',
     default=list,
diff --git a/tests/test-resolve.t b/tests/test-resolve.t
--- a/tests/test-resolve.t
+++ b/tests/test-resolve.t
@@ -400,6 +400,7 @@
   $ hg resolve -l
   R file1
   R file2
+Test option value 'warn'
   $ hg resolve --unmark
   $ hg resolve -l
   U file1
@@ -421,6 +422,26 @@
   $ hg resolve -l
   R file1
   R file2
+If the user passes an invalid value, we treat it as 'none'.
+  $ hg resolve --unmark
+  $ hg resolve -l
+  U file1
+  U file2
+  $ hg --config commands.resolve.mark-check=nope resolve -m
+  (no more unresolved files)
+  $ hg resolve -l
+  R file1
+  R file2
+Test explicitly setting the otion to 'none'
+  $ hg resolve --unmark
+  $ hg resolve -l
+  U file1
+  U file2
+  $ hg --config commands.resolve.mark-check=none resolve -m
+  (no more unresolved files)
+  $ hg resolve -l
+  R file1
+  R file2
 
   $ cd ..