diff --git a/src/python/oftest/controller.py b/src/python/oftest/controller.py
index 2f95a8e..52bd370 100644
--- a/src/python/oftest/controller.py
+++ b/src/python/oftest/controller.py
@@ -592,7 +592,7 @@
         elif issubclass(exp_msg, cfg_ofp.message.message):
             klass = exp_msg
         else:
-            raise ValueError("Unexpected exp_msg argument %r", exp_msg)
+            raise ValueError("Unexpected exp_msg argument %r" % exp_msg)
 
         self.logger.debug("Polling for %s", klass.__name__)