Tween Event + Parameter listener must be non-null

Hi, I am trying to call one tween event after another one has finished. My code is

    mv2 = new Tween(png4_mc,"y", Regular.easeInOut, png4_mc.y,100,0.8,true); 
    mv2.addEventListener(TweenEvent.MOTION_FINISH, play1);
    function play1(event:TweenEvent){
            mv = new Tween(png4_mc,"scaleX", Regular.easeInOut, 0.1, 1,0.5,true);
            mv1 = new Tween(png4_mc,"scaleY", Regular.easeInOut, 0.1, 1,0.5,true);         

This whole thing is happening inside a switch statement. I have also declared mv, mv1 and mv2 outside the switch.

I get the following error

“Parameter listener must be non-null.”

And the event listener never fires the other function…

What am I doing wrong?