diff --git a/radicale/log.py b/radicale/log.py
index 7a49a4e2..bbed5e82 100644
--- a/radicale/log.py
+++ b/radicale/log.py
@@ -55,5 +55,5 @@ def start():
         if debug:
             LOGGER.setLevel(logging.DEBUG)
             LOGGER.debug(
-                "Logging configuration file '%s' not found, using stdout." %
-                filename)
+                "Logging configuration file '%s' not found, using stdout.",
+                extra=filename)