recording from youtube.com with youtube-dl, ffmpeg fails to merge

i am working on a separate, totally different, project involving youtube-dl and ffmpeg. i get only separate video (mp4) and audio (webm) files and an empty that i suspect is the intended merge.

i am trying to download my brother's wedding video at https://www.youtube.com/watch?v=yakmwIqa8cs

i get the follow error messages:

Code:
[download] 100% of 45.58MiB in 01:05
[ffmpeg] Merging formats into "Howard Wedding-yakmwIqa8cs.mp4"
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i 'file:Howard Wedding-yakmwIqa8cs.f137.mp4' -i 'file:Howard Wedding-yakmwIqa8cs.f251.webm' -c copy -map 0:v:0 -map 1:a:0 'file:Howard Wedding-yakmwIqa8cs.temp.mp4'
ERROR: Stream #1:0 -> #0:1 (copy)
Traceback (most recent call last):
File "/usr/local/lib/python3.6/dist-packages/youtube_dl/YoutubeDL.py", line 2075, in post_process
files_to_delete, info = pp.run(info)
File "/usr/local/lib/python3.6/dist-packages/youtube_dl/postprocessor/ffmpeg.py", line 523, in run
self.run_ffmpeg_multiple_files(info['__files_to_merge'], temp_filename, args)
File "/usr/local/lib/python3.6/dist-packages/youtube_dl/postprocessor/ffmpeg.py", line 235, in run_ffmpeg_multiple_files
raise FFmpegPostProcessorError(msg)
youtube_dl.postprocessor.ffmpeg.FFmpegPostProcessorError: Stream #1:0 -> #0:1 (copy)i've also tried merging into mkv with a similar failure. does anyone know what is going on with youtube-dl and ffmpeg? i have gotten merge files from several weeks ago and an attempt back in 2017, but they all have no audio. play this in a browser works fine (i hear audio). playing the webm file plays audio only. thus, i suspect youtube-dl doesn't know how to record all youtube.com videos. how is this one different if it can record other videos?

and, no, i am not in it. so don't bother trying to guess who i am.