Null object reference

I’m creating a movie clip button and for the out state I have this:

web_btn.addEventListener(MouseEvent.MOUSE_OUT, at_out);
function at_out(event:MouseEvent):void {
web_btn.gotoAndPlay(30-(web_btn.currentFrame-11));
}

It gives me this error when I click the button:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at index_fla::web_mc_37/at_out()

I know the error is in this statement:

web_btn.gotoAndPlay(30-(web_btn.currentFrame-11));

But that line of code works fine!

What is it? :eyeup: