Свой компонент не работает

[miki]

<?xml version="1.0" encoding="utf-8"?>
<mx:HBox xmlns:fx="http://ns.adobe.com/mxml/2009" 
		 xmlns:s="library://ns.adobe.com/flex/spark" 
		 xmlns:mx="library://ns.adobe.com/flex/mx">
	<fx:Script>
		<![CDATA[
			public var _a:String = "default";
			public var _b:String = "default";
 
			public function get a():String
			{
				return _a;
			}		
			public function set a( newValue:String ):void
			{
				if (savebtn) savebtn.label = newValue;
				trace(newValue);
				trace(this.savebtn);
			}
			public function get b():String
			{
				return _a;
			}				
			public function set b( newValue:String ):void
			{
				if (loadbtn) loadbtn.label = newValue;
				trace(newValue);
				trace(this.loadbtn);
			}			
		]]>
	</fx:Script>
	<mx:Button id="savebtn" label="{a}" />
	<mx:Button id="loadbtn" label="{b}" />	
</mx:HBox>

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
			   xmlns:s="library://ns.adobe.com/flex/spark" 
			   xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600"
			   xmlns:local="*">
	<local:LastWinner id="my" a="Load" b="Save" />
	<mx:Button x="10" y="30" label="Click" click="my.b = 'new'" />
</s:Application>

Почему кнопки не меняют label на Load и Save ?