diff --git a/youtube_dl/extractor/youtube.py b/youtube_dl/extractor/youtube.py
index 56957a661..6171df84a 100644
--- a/youtube_dl/extractor/youtube.py
+++ b/youtube_dl/extractor/youtube.py
@@ -2435,6 +2435,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
                 subtitles = {}
                 for caption_track in traverse_obj(pctr, (
                         'captionTracks', lambda _, v: v.get('baseUrl'))):
+                    base_url = self._yt_urljoin(caption_track['baseUrl'])
                     if not base_url:
                         continue
                     if caption_track.get('kind') != 'asr':