diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -197,6 +197,9 @@ coreconfigitem('experimental', 'editortm
 coreconfigitem('experimental', 'maxdeltachainspan',
     default=-1,
 )
+coreconfigitem('experimental', 'mmapindexthreshold',
+    default=None,
+)
 coreconfigitem('experimental', 'nonnormalparanoidcheck',
     default=False,
 )
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -605,7 +605,7 @@ class localrepository(object):
         if 0 <= chainspan:
             self.svfs.options['maxdeltachainspan'] = chainspan
         mmapindexthreshold = self.ui.configbytes('experimental',
-                                                 'mmapindexthreshold', None)
+                                                 'mmapindexthreshold')
         if mmapindexthreshold is not None:
             self.svfs.options['mmapindexthreshold'] = mmapindexthreshold