Embedding subtitles fails with invalid argument, appears to be missing --format switch

[pdxbrit]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2019.04.30
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

youtube-dl --verbose -f mp4/best -o Maigret-s01-e01 --write-sub --embed-subs https://www.itv.com/hub/maigret/2a4244a0001
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'--verbose', u'-f', u'mp4/best', u'-o', u'Maigret-s01-e01', u'--write-sub', u'--embed-subs', u'https://www.itv.com/hub/maigret/2a4244a0001']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.04.30
[debug] Python version 2.7.13 (CPython) - Linux-4.14.30-v7+-armv7l-with-debian-9.4
[debug] exe versions: ffmpeg 3.2.10-1, ffprobe 3.2.10-1, rtmpdump 2.4
[debug] Proxy map: {}
[debug] Using fake IP 25.217.171.63 (GB) as X-Forwarded-For.
[ITV] 2a4244a0001: Downloading webpage
[ITV] 2a4244a0001: Downloading XML
WARNING: Unable to download XML: HTTP Error 504: Gateway Time-out
[ITV] 2a4244a0001: Downloading JSON metadata
[ITV] 2a4244a0001: Downloading m3u8 information
[info] Writing video subtitles to: Maigret-s01-e01.en.vtt
[debug] Invoking downloader on u'https://itvpnpdotcom-a.akamaihd.net/2-4244-0001-001/18/2/VAR012/2-4244-0001-001_18_2_VAR012.ism/2-4244-0001-001_18_2_VAR012-audio=128000-video=1378238.m3u8'
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 435
[download] Destination: Maigret-s01-e01
[download] 100% of 955.74MiB in 07:09
[debug] ffmpeg command line: ffprobe -show_streams 'file:Maigret-s01-e01'
[ffmpeg] Fixing malformed AAC bitstream in "Maigret-s01-e01"
[debug] ffmpeg command line: ffmpeg -y -loglevel 'repeat+info' -i 'file:Maigret-s01-e01' -c copy -f mp4 '-bsf:a' aac_adtstoasc 'file:Maigret-s01-e01.temp'
[ffmpeg] Embedding subtitles in 'Maigret-s01-e01'
[debug] ffmpeg command line: ffmpeg -y -loglevel 'repeat+info' -i 'file:Maigret-s01-e01' -i 'file:Maigret-s01-e01.en.vtt' -map 0 -c copy -map '-0:s' -map '-0:d' '-c:s' mov_text -map '1:0' '-metadata:s:s:0' 'language=eng' 'file:Maigret-s01-e01.temp'
ERROR: file:Maigret-s01-e01.temp: Invalid argument
Traceback (most recent call last):
  File "/home/mprice/.local/lib/python2.7/site-packages/youtube_dl/YoutubeDL.py", line 2054, in post_process
    files_to_delete, info = pp.run(info)
  File "/home/mprice/.local/lib/python2.7/site-packages/youtube_dl/postprocessor/ffmpeg.py", line 426, in run
    self.run_ffmpeg_multiple_files(input_files, temp_filename, opts)
  File "/home/mprice/.local/lib/python2.7/site-packages/youtube_dl/postprocessor/ffmpeg.py", line 235, in run_ffmpeg_multiple_files
    raise FFmpegPostProcessorError(msg)
FFmpegPostProcessorError: file:Maigret-s01-e01.temp: Invalid argument

Description

When trying to embed subtitles, the script fails with an Error: Invalid argument error on the output file. Taking the the ffmpeg command line arguments from the verbose output, and running it again independently generates a more verbose error of: [NULL @ 0x1a46040] Unable to find a suitable output format for 'file:Maigret-s01-e01.temp'
file:Maigret-s01-e01.temp: Invalid argument

ffmpeg -y -loglevel repeat+info -i file:Maigret-s01-e01.mp4 -i file:Maigret-s01-e01.en.vtt -map 0 -c copy -map -0:s -map -0:d -c:s mov_text -map 1:0 -metadata:s:s:0 language=eng file:Maigret-s01-e01.temp
ffmpeg version 3.2.10-1~deb9u1+rpt1 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 6.3.0 (Raspbian 6.3.0-18+rpi1) 20170516
  configuration: --prefix=/usr --extra-version='1~deb9u1+rpt1' --toolchain=hardened --libdir=/usr/lib/arm-linux-gnueabihf --incdir=/usr/include/arm-linux-gnueabihf --enable-gpl --disable-stripping --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libebur128 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmp3lame --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx-rpi --enable-mmal --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared
  libavutil      55. 34.101 / 55. 34.101
  libavcodec     57. 64.101 / 57. 64.101
  libavformat    57. 56.101 / 57. 56.101
  libavdevice    57.  1.100 / 57.  1.100
  libavfilter     6. 65.100 /  6. 65.100
  libavresample   3.  1.  0 /  3.  1.  0
  libswscale      4.  2.100 /  4.  2.100
  libswresample   2.  3.100 /  2.  3.100
  libpostproc    54.  1.100 / 54.  1.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'file:Maigret-s01-e01.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.56.101
  Duration: 01:26:54.08, start: 0.000000, bitrate: 1482 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 896x504 [SAR 1:1 DAR 16:9], 1378 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 96 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
Input #1, webvtt, from 'file:Maigret-s01-e01.en.vtt':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Subtitle: webvtt
[NULL @ 0x1a46040] Unable to find a suitable output format for 'file:Maigret-s01-e01.temp'
file:Maigret-s01-e01.temp: Invalid argument

Adding a -f mp4 switch prior to the output filename appears to resolve the issue for me.

Is this a bug, or am I missing a switch on my original youtub-dl command line? youtube-dl --verbose -f mp4/best -o Maigret-s01-e01 --write-sub --embed-subs https://www.itv.com/hub/maigret/2a4244a0001

[dstftw]

Don't touch -o if you have no idea what you are doing.