script controller

hi

could somebody tell me how can i lock an object's Z position with a script controller attached to the Z Position track in the motion panel?

I want the object not to go below 0.0 in Z. I'm using this:

if ($.pos.z < 0.0) then return 0.0
$.pos.z

it works, but every time a release the object and pickit again to move its jumps to 0.0 in Z. just try it. why is this hapening!??

thanks