Error #1034: Type Coercion failed ?

Hi there,

I'm using Action Script 3 and CS4

I'm trying to create a product and when i play it I get this error:

TypeError: Error #1034: Type Coercion failed: cannot convert flash.display::MovieClip@16ed7e1 to flash.display.SimpleButton.

at flash.display::MovieClip/gotoAndPlay()
at Homepage_fla::MainTimeline/after()[Homepage_fla.MainTimeline::frame88:5]

The coding for the scene is:

stop();

function Goto (event:MouseEvent): void {
gotoAndPlay (1,"The Course");
}
function GoBack(event:MouseEvent): void {
gotoAndPlay (1,"Aperture");
}
function Home(event:MouseEvent): void {
gotoAndPlay (1,"Homepage");
}

btn_bac.addEventListener(MouseEvent.CLICK, GoBack);

btn_thecourse.addEventListener(MouseEvent.CLICK, Goto);

btn_homepagee.addEventListener(MouseEvent.CLICK, Home);