title: Could not find tag for codec subrip in stream, codec not currently supported in container
body: <p>I'm trying to convert a video to lower quality using the following comand :</p>
  
  <blockquote>
    <p><code>ffmpeg -i in.mkv -c:a copy -map 0:5 -map 0:6 -map 0:7 -c:s copy -map 0:9 -c:v h264_nvenc -rc constqp -qp 23 -map 0:0 out.mp4</code></p>
  </blockquote>
  
  <p>With stream 0:9 being a subtitle. I get</p>
  
  <pre><code>Stream mapping:
    Stream #0:5 -&gt; #0:0 (copy)
    Stream #0:6 -&gt; #0:1 (copy)
    Stream #0:7 -&gt; #0:2 (copy)
    Stream #0:9 -&gt; #0:3 (copy)
    Stream #0:0 -&gt; #0:4 (h264 (native) -&gt; h264 (h264_nvenc))
  Press [q] to stop, [?] for help
  [mp4 @ 0x55c6edd3a640] track 0: codec frame size is not set
  [mp4 @ 0x55c6edd3a640] track 1: codec frame size is not set
  [mp4 @ 0x55c6edd3a640] track 2: codec frame size is not set
  [mp4 @ 0x55c6edd3a640] Could not find tag for codec subrip in stream #3, codec not currently supported in container
  Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
  Error initializing output stream 0:4 -- 
  </code></pre>
  
  <p>If I change <code>-c:s copy</code> with <code>-c:s mov_text</code>, the error is gone, but then the subtitle doesn't work. Everything else seems to be fine.</p>