ReferenceError: Error #1069: 在 spark.components.RadioButtonGroup 上找不到属性 label，且没有默认值

1、错误描述

ReferenceError: Error #1069: 在 spark.components.RadioButtonGroup 上找不到属性 label，且没有默认值。
	at Chart/radiogroup_itemClickHandler()[E:Flash BuilderMapsrcChart.mxml:49]
	at Chart/__radiogroup_itemClick()[E:Flash BuilderMapsrcChart.mxml:58]
	at flash.events::EventDispatcher/dispatchEventFunction()
	at flash.events::EventDispatcher/dispatchEvent()
	at spark.components::RadioButton/buttonReleased()[E:dev4.0.0frameworksprojectssparksrcsparkcomponentsRadioButton.as:554]
	at spark.components.supportClasses::ButtonBase/mouseEventHandler()[E:dev4.0.0frameworksprojectssparksrcsparkcomponentssupportClassesButtonBase.as:1054]

protected function radiogroup_itemClickHandler(event:ItemClickEvent):void
{
	if(event.currentTarget.label=="周")
	{
		Alert.show("OK!");
	}
}

<fx:Declarations>
	<s:RadioButtonGroup />
</fx:Declarations>