--- a/mercurial/configitems.py	Wed Oct 11 23:33:59 2017 +0200
+++ b/mercurial/configitems.py	Fri Oct 13 23:33:31 2017 +0200
@@ -18,7 +18,7 @@
 def loadconfigtable(ui, extname, configtable):
     """update config item known to the ui with the extension ones"""
     for section, items in configtable.items():
-        knownitems = ui._knownconfig.setdefault(section, {})
+        knownitems = ui._knownconfig.setdefault(section, itemregister())
         knownkeys = set(knownitems)
         newkeys = set(items)
         for key in sorted(knownkeys & newkeys):