Can't stop setInterval with clearInterval

var mySeconds = 5;
this.seconds_txt.text = mySeconds;
var timerId = setInterval(countDown.bind(this), 1000);

function countDown(){
if(mySeconds == 0){

   this.myStopIntervalFunction(); //call to function fires but doesn't clear interval
	clearInterval(countDown); //doesn't work

}
else{
     mySeconds--;
     this.seconds_txt.text = mySeconds;
     }
}

this.myStopIntervalFunction = function (root) {
	console.log("myStopFunction fired");
	root.clearInterval(countDown);
};

I have no idea why its not working.