SWF LOADER null pointer exception

Question

Hello,
I an trying to load and stop a SWF video.
The video loads perfectly but when I click on stop, it gives me a null pointer exception.
The thing is this, in STOP, the variable asociation:
                var mc : MovieClip = loader.content as MovieClip;
gives me as a result null (mc:null) , so when I excecute  mc.stop(); it gives me a null pointer exception.
Why is mc null? if the content exists and loads?
thank you VERY MUCH!
Agustina
CODE
<mx:Canvas xmlns:mx="http://www.adobe.com/2006/mxml" width="1280" height="720" xmlns:views="code.views.*" xmlns:controllers="code.controllers.*" backgroundColor="#000000" creationComplete="OnCreationComplete()">
          <mx:Script>
          <![CDATA[
                    import mx.core.UIComponent;
                    private var loader:Loader;
                    private function OnCreationComplete():void
                              var url : String ="flashvideos/Advertising.swf";
                              loader = new Loader();
                              var request:URLRequest = new URLRequest(url);
                              loader.load(request);
                              loaderHolder.rawChildren.addChild(loader as DisplayObject);
                    private function onStop():void
                         var mc : MovieClip = loader.content as MovieClip;
                         mc.stop();
                    private function onPlay():void
                         var mc : MovieClip = loader.content as MovieClip;
                         mc.play();
]]>
          </mx:Script>
<mx:VBox width="100%" height="100%" horizontalAlign="center" verticalAlign="middle">
                    <mx:Canvas id="loaderHolder" width="500" height="500"/>
                         <mx:Button label="Stop" click="onStop()"/>
                         <mx:Button label="Play" click="onPlay()"/>
</mx:VBox>
</mx:Canvas>

Answer 1

If the swf is not an AS3/FP9 SWF (or newer) it will not be a MovieClip.