TypeError?

I have a movie-clip with a roll-over,roll-out and on-click function; a pretty simple button. When I test my movie the button works just fine and when it is clicked the movie continues. However, when I click the button an error messages also appears. Below is the error I receive and the code applied to my movie-clip. Any thoughts on this error message?

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

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stop();

start_btn.buttonMode = true;

start_btn.addEventListener(MouseEvent.ROLL_OVER, rollOverFunc);
start_btn.addEventListener(MouseEvent.ROLL_OUT, rollOutFunc);
start_btn.addEventListener(MouseEvent.CLICK, clickFunc);

function rollOverFunc(event:MouseEvent):void
{
start_btn.gotoAndPlay(2);
}

function rollOutFunc(event:MouseEvent):void
{
start_btn.gotoAndPlay(7);
}

function clickFunc(event:MouseEvent):void
{
play();
}