--- a/mercurial/dispatch.py	Sun May 20 17:35:57 2018 +0530
+++ b/mercurial/dispatch.py	Sun May 20 17:37:07 2018 +0530
@@ -288,8 +288,8 @@
                 req.args[2] != 'serve' or
                 req.args[3] != '--stdio'):
                 raise error.Abort(
-                    _('potentially unsafe serve --stdio invocation: %r') %
-                    (req.args,))
+                    _('potentially unsafe serve --stdio invocation: %s') %
+                    (stringutil.pprint(req.args),))
 
         try:
             debugger = 'pdb'