How to display elapsed/current time of a song in Actionscript.

Hi I have a question on how I can make my code display a total time for a song from a media player I created with Actionscript 3.0, using XML to load the song track.

This is my current function which I have done and I am unsure on what the problem is that is not allowing the elapsed time to display in a textfield on my stage.

stage.addEventListener(Event.ENTER_FRAME, timeElapsed);

function timeElapsed(e:Event):void

{

                    var minutesCurr:Number = Math.floor((my_channel.position /1000) /60);

                    var secondsCurr = Math.floor((my_channel.position /1000) % 60);

        if (secondsCurr < 10)

                    {

            secondsCurr = "0" + secondsCurr;

        }

        var minutesTot:Number = Math.floor((my_sound.length /1000) /60);

        var secondsTot = Math.floor((my_sound.length /1000) % 60);

        if (secondsTot < 10)

                    {

            secondsTot = "0" + secondsTot;

        }

        completeText_txt.text = minutesCurr + ":" + secondsCurr + "/" + minutesTot + ":" + secondsTot;

}

I tried adding to the stage and I am getting a null object reference.

Thanks Casey