Verticle mouse_move scrolling

programming flash
1

Can someone explain to me why it scrolls down fine but when it scrolls up the first line of text falls to the middle of the pageContainer?

the height of the pageContainer is 440 pixels.

I cant get it to stop at the top of the pageContainer.It just keeps going up (into blank space)and the text keeps moving.

Heres the code:

pageContainer.addEventListener(MouseEvent.MOUSE_MOVE, scrollPage);

function scrollPage(event:MouseEvent):void
{

TweenLite.to(pageContainer, 0.5, { y:-(mouseY/440)*pageContainer.height+stage.stageHeight/2});

}

Thanks for your help.