Could someone tell me why this doesnt work? (swapDepth)

could someone help me to figure out this, and how to code this correctly, it’s probably something simple though right now I can’t figure it out!

I want the circle to swapdepth with the box when the circle._y is behind the box._y

steer the circle with the arrow keys, the actions are in the first frame of each movie clip.

thanx in advance