FFmpeg text at random position

I need to print text at random position (to protect videos).

The following work, but it seems the random position is updated at miliseconds, what makes the text go crazy across the screen. Someone has any idea? Thanks

ffmpeg -i video.mov -vf "drawtext=fontfile=/Windows/Fonts/arial.ttf: text='Test Text':x=random(1)*w: y=random(1) * h" -acodec copy output.avi