ffmpeg error: input packet contains no samples av_interleaved_write_frame()

I use ffmpeg to concat lots and lots of clips – that works perfectly fine in 99% of the cases. That said, sometimes I get this error:
(mov @ 0x7fcc94004000) fatal error, input packet contains no samples
av_interleaved_write_frame(): Not yet implemented in FFmpeg, patches welcome

The output is still produced and looks fine – but I really would like to understand what throws this error. I’m pretty sure it’s a problem with the audio – all clips have audio in the same format (PCM, little endian, 48 kHz, 16 bit), but one clip might have stereo, the other mono, and it only happens when mixing these. I have this all the time though and usually ffmpeg does not complain, so it must be something else. Any ideas?

Just for reference (I don’t think the line is the problem, but…), this is how I call ffmpeg (from Python):
subprocess.call(('ffmpeg', '-f', 'concat', '-safe', '0', '-i', path_concatlist, '-c:v', 'copy', '-c:a', 'copy', output))