diff --git a/youtube_dl/casefold.py b/youtube_dl/casefold.py
index 748c2d491..ad9c66f8e 100644
--- a/youtube_dl/casefold.py
+++ b/youtube_dl/casefold.py
@@ -1663,5 +1663,5 @@ def casefold(s):
 
 
 __all__ = [
-    casefold
+    'casefold',
 ]