ActionScript 3.0 :: Hide White Background In CellRenderer Cells?

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:[url]......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;
}
 
1119: Access of possibly undefined property skin through a reference with static type MarksCellRenderer.
 
how to hide the defualt white cell background?