where does "0kB" come from?

ffmpeg -re -i "$v" -t 1 -vcodec copy -acodec copy -f flv -y v.flv 2>&1|tail -2|head -1|tee tee.log|cut -d'=' -f5; cat tee.log
       0kB time
frame=   25 fps= 13 q=-1.0 Lsize=     254kB time=00:00:00.95 bitrate=2175.6kbits/s