Can't change _alpha

[marinebro0306]

There's probably some stupid mistake I'm doing here, but I cant get this to work.
I have a circle thingy called "shape". I also have a button that says "More Transparent," which is supposed to make the circle's transparency to 30%. The button's script is:
on(release) {
setProperty("shape", _alpha, "30");
}

But, it does nothing when clicked...