Loading .swf, addChild, and removeChild

Hello all,

I'm trying load an swf when you click a button, and load another in it's place when you click a different button.   I'm using addChild to insert the swf onto the stage.  Problem is that every time I click a new button, the new swf is loaded on top of the previous one, and I need the previous one to disappear.  I tried using the removeChild method before the new swf is loaded to no effect.

Here's the function I'm using for the loader with my removeChild attempt in bold:

function newPage():void {
    trace("load activated");
    trace(""+target+".swf");
    var req:URLRequest=new URLRequest(""+target+""+".swf");
    var loader:Loader = new Loader();
   if (loaderState==true){
        top_mc.removeChild(loader);
    }
    loader.load(req);
    loader.contentLoaderInfo.addEventListener(ProgressEvent.PROGRESS, preLoad);
    loader.contentLoaderInfo.addEventListener(Event.COMPLETE, fileLoaded);
    function preLoad(event:ProgressEvent):void {
        var percent:Number=Math.round(event.bytesLoaded/event.bytesTotal*100);
        top_mc.preload_txt.text=String(""+percent+"");
    }
    function fileLoaded(event:Event):void {
        trace("file loaded");
        top_mc.addChild(loader);
        loaderState=true;
        }
}

The error I get is as follows:

ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.
    at flash.display::DisplayObjectContainer/removeChild()
    at main_fla::MainTimeline/newPage()
    at main_fla::MainTimeline/workLoaderEnter()
    at main_fla::MainTimeline/clickF()

Perhaps there is a way to give the addChild an instance name so that I can remove it later?

Any ideas?

Thanks!