diff --git a/youtube_dl/YoutubeDL.py b/youtube_dl/YoutubeDL.py
index fca4999eb5..ad35988052 100755
--- a/youtube_dl/YoutubeDL.py
+++ b/youtube_dl/YoutubeDL.py
@@ -1853,7 +1853,7 @@ class YoutubeDL(object):
                     def compatible_formats(formats):
                         video, audio = formats
                         # Check extension
-                        video_ext, audio_ext = audio.get('ext'), video.get('ext')
+                        video_ext, audio_ext = video.get('ext'), audio.get('ext')
                         if video_ext and audio_ext:
                             COMPATIBLE_EXTS = (
                                 ('mp3', 'mp4', 'm4a', 'm4p', 'm4b', 'm4r', 'm4v', 'ismv', 'isma'),