ActionScript 3.0 :: Scale9grid A Button To Preserve Its Rounded Corners When Resizing?

I am trying to scale9grid a button to preserve its rounded corners when resizing but I have run into trouble.The button is an ordinary Button symbol and it has 4 layered states (Up, Down, Over, Hit).In each layer there are some shapes (the outer button border, two background shapes inside the button for shading and then some extra outer border shapes to create the pressed down effect). The button contains no other elements,not even a textfield (the textfield is on the same level as the button).

The button is inside a movieclip (which also carries the textfield) that is of the same size as the button and is resized as by Adobe design when the button is resized (i.e. I am never manipulating its size).The Button is 35 high and the textfield is 25 high. They have their registration points both centered and placed at the the same coordinate.I use the follow code to resize and scale9grid the button:

popup.btnLoginButton.width = popup.btnLoginText.width + 20;
popup.btnLoginButton.scale9Grid = new Rectangle(10, 5, popup.btnLoginText.width, 15);

The above scale9Grid throws a Error #2004: One of the parameters is invalid.The same error occurs unless the Rectangle parameters are very small - for example Rectangle(5, 5, 5, 5) even though the textfield is much longer. But when the Rectangle is that small of course no scale9grid scaling is taking place.