Output Issues with AS buttons

Hi,

Don’t know much about AS but I have tried to work my way through it.
I can’t understand why my button actions are causing an output error,
the first two on this keyframe work just fine, but when I add the third it causes
this error…

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at clear_fla::MainTimeline/frame86()

Here is the script

stop ();

function clickFunction(evt:MouseEvent):void {
gotoAndPlay(87, “Scene 1” );
}
ak_btn.addEventListener(MouseEvent.CLICK, clickFunction);

function clickGrap(evt:MouseEvent):void {
gotoAndPlay(145, “Scene 1” );
}
sg_btn.addEventListener(MouseEvent.CLICK, clickGrap);

function clickTrav(evt:MouseEvent):void {
gotoAndPlay(153, “Scene 1” );
}
az.addEventListener(MouseEvent.CLICK, clickTrav);

Like I said, sg_btn and ak_btn work, but az causes the error…
They are all on the same keyframe, dont know if that matters

If anybody can help me out it is appreciated.

Mike