Hide white background in CellRenderer cells

Hi

I am developing an application and making use of a list component. I have set the List to use a custom cellrenderer.

I need the list to have rounded corners but the problem is that where my custom cell corners are transparent I can see a default white cell underneath instead of the background.

I found this discussion seemed to be relevant http://supportforums.blackberry.com/t5/Tablet-OS-SDK-for-Adobe-AIR/TileList-CellRenderer-has-white-background/td-p/781843

But when I add this function to my cellrenderer class I get  compiler error

override public function set data(data:Object):void{

    super.data = data;

    // Set the default skin to be invisible. This is a bit

    // easier than creating a new skin, if your goal is

    // to hide the white background.

    this.skin.visible = false;

}

Can anyone tell me how to hide the defualt white cell background?

Thanks

Graeme