Error #1010 on a button

I have a button on frame 2.

stop();

easy_btn.addEventListener(MouseEvent.CLICK, onEasy); function onEasy(event:MouseEvent):void { gotoAndStop(4) }

When I click on it, it gives this in the output:

"TypeError: Error #1010: A term is undefined and has no properties. at SpaceInvaders_fla::MainTimeline/frame4() at flash.display::MovieClip/gotoAndStop() at SpaceInvaders_fla::MainTimeline/onEasy()"

I don't understand why it is doing this.