--command flag fails with "supported commands are"...

[vredesbyyrd]

Hello, thanks for sharing this awesome project.

I am trying to use --command to transcode video that is not supported by default.

mkchromecast --debug --video --command 'ffmpeg -re -i "/mnt/TARS/test.mkv" -c:v libx264 -preset ultrafast -tune zerolatency -c:a aac -f mp4 -movflags frag_keyframe+empty_moov pipe:1'

It fails for the most part silently, only the following is returned:

Supported commands are: 
- ffmpeg
- youtube-dl

I can cast supported formats that do not require transcoding as well as --screencast without issue. Any thoughts on what could be happening here?

Thanks for your time

EDIT:

mkchromecast version: v0.3.9 (built from master) running on linux