diff -r 0314e02efa25 -r ca5b833ce756 mercurial/localrepo.py
--- a/mercurial/localrepo.py	Sun Oct 08 20:42:19 2017 +0200
+++ b/mercurial/localrepo.py	Sun Oct 08 20:43:46 2017 +0200
@@ -605,7 +605,7 @@
         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