[FFmpeg-user] h264_v4l2m2m won't write to .mkv

[Neale Pickett]

Can anyone help me understand why the h264_v4l2m2m codec won't write to a
mkv container, but will write to a nut which can then be converted to mkv?
Is there a way I can skip this intermediate step and write directly to mkv?

sweetums:/home/neale/tmp % ffmpeg -y -hide_banner -loglevel warning -i
ElephantsDream.mp4 -to 00:00:02 -c:v h264_v4l2m2m out.mkv
[h264_v4l2m2m @ 0xaaaae376e5d0] Failed to set gop size: Invalid argument
Could not write header for output file #0 (incorrect codec parameters ?):
Invalid data found when processing input
Error initializing output stream 0:0 --
[libvorbis @ 0xaaaae379ea70] 39 frames left in the queue on closing
sweetums:/home/neale/tmp % ffmpeg -y -hide_banner -loglevel warning -i
ElephantsDream.mp4 -to 00:00:02 -c:v h264_v4l2m2m out.nut
[h264_v4l2m2m @ 0xaaab14fb8f50] Failed to set gop size: Invalid argument
[nut @ 0xaaab14fbcd50] Non-monotonous DTS in output stream 0:0; previous:
0, current: 0; changing to 1. This may result in incorrect timestamps in
the output file.
sweetums:/home/neale/tmp % ffmpeg -y -hide_banner -loglevel warning -i
out.nut -map 0 -c copy out.mkv
[h264 @ 0xaaaaf15047b0] no frame!
sweetums:/home/neale/tmp %