--- a/contrib/debugshell.py	Wed Jun 22 21:30:49 2016 +0100
+++ b/contrib/debugshell.py	Tue Jun 21 00:50:39 2016 +0900
@@ -52,7 +52,7 @@
         with demandimport.deactivated():
             __import__(pdbmap[debugger])
     except ImportError:
-        ui.warn("%s debugger specified but %s module was not found\n"
+        ui.warn(("%s debugger specified but %s module was not found\n")
                 % (debugger, pdbmap[debugger]))
         debugger = 'pdb'