[flexcoders] setStyle not valid

[j2me_soul]

I try to change the style at runtime 
 
 <mx:Button x="38" y="44" label="为中国喝彩" click="butClickHandler(event)"/>
 <mx:Label id="styleLabel" text="I'm Label"/>

   private function butClickHandler(event:MouseEvent):void
   {
        /* it doesn't work */
        styleLabel.setStyle("font-size", 72);
   }