Movie Clip Button problem

[Visions of them]

Hi
I'm using the following code

portfolio_mc.onRelease=function(){
portfolio_mc.gotoAndStop(10);
}

and I keep getting the message

Scene=Scene 1, Layer=backdrop, Frame=1: Line 1: Statement must appear within on/onClipEvent handler
portfolio_mc.onRelease=function(){

but I can not figure out whats wrong, i have given the clip an instance name

thanks in advance