diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index ef0afd686..e722eed58 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -1830,7 +1830,7 @@ def write_json_file(obj, fn):
 
     try:
         with tf:
-            json.dump(obj)
+            json.dump(obj, tf)
         if sys.platform == 'win32':
             # Need to remove existing file on Windows, else os.rename raises
             # WindowsError or FileExistsError.