Ffmpeg error: cur_dts is not valid

Question

I am trying to record video and audio from a webcam with the following command:

ffmpeg -f v4l2 -input_format mjpeg -s uhd2160 -i /dev/video0 -f pulse -channels 1 -i default video.mpg

but it seems to be stuck in an infinite loop at the bottom of the output, which looks like this:

cur_dts is invalid (this is harmless if it occurs once at the start per stream)
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
...

Full output with flag -reportenabled: https://pastebin.com/bV1ePJ2u

I can record video and audio separately simply:

ffmpeg -f v4l2 -input_format mjpeg -s uhd2160 -i /dev/video0 video.mpg
ffmpeg -f pulse -channels 1 -i default audio.wav

So what could be the problem here? I'm on Ubuntu 16.04 kernel 4.14.1