diff --git a/lib/middleware/karma.js b/lib/middleware/karma.js
index 358e29ab7..c51c34491 100644
--- a/lib/middleware/karma.js
+++ b/lib/middleware/karma.js
@@ -181,7 +181,7 @@ function createKarmaMiddleware (
             const fileType = file.type || path.extname(filePath).substring(1)
 
             if (helper.isDefined(fileType) && !FILE_TYPES.includes(fileType)) {
-              log.warn('Invalid file type, defaulting to js.', fileType)
+              log.warn(`Invalid file type (${fileType}), defaulting to js.`)
             }
 
             if (!file.isUrl) {