Raspberry Pi 4, h264_v4l2m2m hardware encoder support not working as expected

[charkster]

Hi,
I'm trying to get my Raspberry Pi 4's hardware h264 encoder to work with motion. Command line ffmpeg supports the h264_v4l2m2m and h264_omx encoders, but when I specify that encoder in the motion.conf file:
movie_codec mp4:h264_v4l2m2m

I see errors in the logfile:
[1:ml1] [NTC] [ENC] [Sep 28 21:00:35] ffmpeg_set_codec_preferred: Using codec h264_v4l2m2m
[1:ml1] [ERR] [ENC] [Sep 28 21:00:35] ffmpeg_set_codec: Could not open codec Invalid argument

I am using a "RPi Camera Module 3" camera connected with the mipi cable. I start motion with this command:
libcamerify motion

motion Version 4.5.1