Merge 943b05300ac6043916722fe0e4897ba8fb6cdd67 into 3eb8d22ddb8982ca4fb56bb7a8d6517538bf14c6

README.md

@@ -948,7 +948,7 @@ When youtube-dl detects an HLS video, it can download it either with the built-i
 
 When youtube-dl knows that one particular downloader works better for a given website, that downloader will be picked. Otherwise, youtube-dl will pick the best downloader for general compatibility, which at the moment happens to be ffmpeg. This choice may change in future versions of youtube-dl, with improvements of the built-in downloader and/or ffmpeg.
 
-In particular, the generic extractor (used when your website is not in the [list of supported sites by youtube-dl](https://ytdl-org.github.io/youtube-dl/supportedsites.html) cannot mandate one specific downloader.
+In particular, the generic extractor (used when your website is not in the [list of supported sites by youtube-dl](https://ytdl-org.github.io/youtube-dl/supportedsites.html)) cannot mandate one specific downloader.
 
 If you put either `--hls-prefer-native` or `--hls-prefer-ffmpeg` into your configuration, a different subset of videos will fail to download correctly. Instead, it is much better to [file an issue](https://yt-dl.org/bug) or a pull request which details why the native or the ffmpeg HLS downloader is a better choice for your use case.
 

youtube_dl/jsinterp.py

@@ -686,6 +686,8 @@ class JSInterpreter(object):
             raise self.Exception('Cannot get index {idx!r:.100}'.format(**locals()), expr=repr(obj), cause=e)
 
     def _dump(self, obj, namespace):
+        if obj is JS_Undefined:
+            return 'undefined'
         try:
             return json.dumps(obj)
         except TypeError: