diff -r d2ac8b57a75d -r 41dcd7545266 mercurial/ui.py
--- a/mercurial/ui.py	Wed Feb 24 10:41:15 2016 -0800
+++ b/mercurial/ui.py	Wed Feb 24 15:55:44 2016 -0600
@@ -796,7 +796,7 @@
         If ui is not interactive, the default is returned.
         """
         if not self.interactive():
-            self.write(msg, ' ', default, "\n")
+            self.write(msg, ' ', default or '', "\n")
             return default
         try:
             r = self._readline(self.label(msg, 'ui.prompt'))