Ffmpeg error: amv files only support 2 streams

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I'm using ffmpeg to convert videos to amv format. Most of the time it works fine.

Sometimes I get the error: amv files only support 2 streams.

How can I fix this?

Thanks

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Here is the script I use:

-c:v amv -c:a adpcm_ima_amv -pix_fmt yuvj420p -vf "scale=320:-1,crop=256:in_h:32:0,pad=320:ih:64:-1:color=black" -vstrict -1 -r 25 -ac 1 -ar 22050 -block_size 882