I trying to close a message box with a okay button inside its movieclip but i keep getting "1119: Access of possibly undefined property btnOk through a reference with static type flash.display:SimpleButton." error message when testing. I not sure whats wrong. Below is my code. anyone please help.

//open message box
btnPassport.addEventListener(MouseEvent.MOUSE_DOWN, btnPassportClicked);
function btnPassportClicked(event:MouseEvent):void {

//close message box wiht okay button within the movieclip.
mcPassport.btnOk.addEventListener(MouseEvent.MOUSE_DOWN, mcPassportOkClicked);
	function mcPassportOkClicked(event:MouseEvent):void {