diff -r 7edf68862fe3 -r d658cbef8041 mercurial/cmdutil.py
--- a/mercurial/cmdutil.py	Sun Apr 08 15:22:30 2018 +0900
+++ b/mercurial/cmdutil.py	Sun Apr 08 15:32:09 2018 +0900
@@ -1398,7 +1398,7 @@
         else:
             # launch the editor
             message = None
-        ui.debug('message:\n%s\n' % message)
+        ui.debug('message:\n%s\n' % (message or ''))
 
         if len(parents) == 1:
             parents.append(repo[nullid])