求帮助

我写了段代码， var a:bmj=new bmj();
tu0_mc.addEventListener(MouseEvent.CLICK,dj);
function dj(cs:MouseEvent) { 
addChild(a);
a.scaleX=2.5;
a.scaleY=2.5;
}
stage.addEventListener(MouseEvent.CLICK,djwt);
function djwt(wtcs:MouseEvent) { 
removeChild(a);
}
测试影片时显示
ArgumentError: Error #2025: 提供的 DisplayObject 必须是调用者的子级。at flash.display::DisplayObjectContainer/removeChild()at _fla::MainTimeline/djwt()
求哪位高手给解释一下，多谢！