diff --git a/src/tox/config/__init__.py b/src/tox/config/__init__.py
index d2fb4d82e..1de6054fc 100644
--- a/src/tox/config/__init__.py
+++ b/src/tox/config/__init__.py
@@ -122,7 +122,7 @@ def add_testenv_attribute_obj(self, obj):
     def parse_cli(self, args, strict=False):
         args, argv = self.argparser.parse_known_args(args)
         if argv and (strict or WITHIN_PROVISION):
-            self.argparser.error("unrecognized arguments: %s".format(" ".join(argv)))
+            self.argparser.error("unrecognized arguments: {}".format(" ".join(argv)))
         return args
 
     def _format_help(self):