title: Invalid audio stream. Exactly one MP3 audio stream is required
body: <p>I've written a small line to concat two mp3 files and add background music to the result. It works quite well, but I'm getting some weird errors. Can anyone identify the cause?</p>
  
  <p>(inside a for loop)</p>
  
  <pre><code>ffmpeg -y -i "Input/%%i" -i "Assets/outro.mp3" -filter_complex "[0:a][1:a]concat=n=2:v=0:a=1;amovie='Assets/music.mp3':loop=999,volume=0.2[s];[0][s]amix=duration=shortest" "tmp.audio.mp3"
  </code></pre>
  
  <p>Errors:</p>
  
  <p>[mp3 @ 000000000059f180] Invalid audio stream. Exactly one MP3 audio stream is required.
  Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
  Error initializing output stream 0:1 -- </p>
  
  <p>Log file: <a href="https://pastebin.com/PLnu0Fvn" rel="nofollow noreferrer">https://pastebin.com/PLnu0Fvn</a></p>