How to fork ffmpeg x11grab?

I'm trying to fork x11grab to no avail. m.mp4 is not generated:

ffmpeg -video_size 1024x768 \
       -framerate 25 \
       -f x11grab \
       -i :0.0+100,100 \
       /tmp/m.mp4 \
>> /tmp/l.log 2>&1 &

Only after removing the trailing ampersand & , m.mp4 is generated. Any ideas what I'm missing?