error 1009 buttons

I have used the following code:

stop();

cap_Btn.addEventListener(
MouseEvent.MOUSE_DOWN,
function(evt:MouseEvent):void {
gotoAndPlay("_cap");
}
);

samp_Btn.addEventListener(
MouseEvent.MOUSE_DOWN,
function(evt:MouseEvent):void {
gotoAndPlay("_samp");
}
);

and received this error when I click on the button:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at MethodInfo-63()

What am I doing wrong?

Thanks in advance for any help with this issue.

Rob