diff --git a/Lib/test/test_urllibnet.py b/Lib/test/test_urllibnet.py
index e99428f4105..b811930bbc7 100644
--- a/Lib/test/test_urllibnet.py
+++ b/Lib/test/test_urllibnet.py
@@ -185,7 +185,7 @@ def test_data_header(self):
             try:
                 time.strptime(datevalue, dateformat)
             except ValueError:
-                self.fail('Date value not in %r format', dateformat)
+                self.fail('Date value not in %r format' % dateformat)
 
     def test_reporthook(self):
         records = []