diff --git a/pydle/features/rfc1459/client.py b/pydle/features/rfc1459/client.py
index f23ba0f..fc81099 100644
--- a/pydle/features/rfc1459/client.py
+++ b/pydle/features/rfc1459/client.py
@@ -970,7 +970,7 @@ def on_raw_422(self, message):
 
     def on_raw_421(self, message):
         """ Server responded with 'unknown command'. """
-        self.logger.warning('Server responded with "Unknown command: %s"'.format(message.params[0]))
+        self.logger.warning('Server responded with "Unknown command: %s"', message.params[0])
 
     def on_raw_432(self, message):
         """ Erroneous nickname. """