mercurial/ui.py

--- a/mercurial/ui.py	Wed Oct 11 05:18:20 2017 +0200
+++ b/mercurial/ui.py	Wed Oct 11 05:36:10 2017 +0200
@@ -1149,7 +1149,7 @@
             defaultinterface = i
 
         choseninterface = defaultinterface
-        f = self.config("ui", "interface.%s" % feature, None)
+        f = self.config("ui", "interface.%s" % feature)
         if f in availableinterfaces:
             choseninterface = f