Changing Movie Clip Properties

I have a movie clip on the stage called ball_mc. I have a button called yscale_btn with an input text field next to it called yscale_txt. I’d like to take the input value to transform the scaleY property of ball_mc. Here’s my code:

var yscale_txt:Number;

yscale_btn.addEventListener(MouseEvent.CLICK, yScale);
function yScale(event:MouseEvent):void {
    ball_mc.scaleY = yscale_txt;

There are no compile or script errors. When I test the movie though, any value that I put into the text field, even 0.0002, makes the scaleY so great that the clip goes off the stage. What am I doing wrong here?