s:SpriteVisualElement and "component declarations are not allowed here"

Take a look at the following code.

<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" 
                xmlns:view="client.view.*" minWidth="955" minHeight="600">
     <s:Panel x="10" y="10" height="100%" title="CTW" borderColor="#008040" fontFamily="Arial" fontWeight="bold" fontSize="13">
         <s:HGroup>
             ...
             <s:Scroller id="canvasGroup" width="650" height="500">
                 <s:Group>
                     <s:SpriteVisualElement>
                         <view:PNGCanvas id="canvas" />  <!-- error is thrown here -->
                     </s:SpriteVisualElement>
                 </s:Group>
             </s:Scroller>
         </s:HGroup>
     </s:Panel>  
 </s:Application>
 

PNGCanvas extends flash.display.Sprite. I'm getting an error component declarations are not allowed here (on line marked with error is thrown here). What's wrong here?

Thank you in advance!!!

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