Property 'skin' is read only - > Flex Skinning

I want to decorate a flex Button with a cutom skin component .

But when i point the 'skin' property to the source , it says "Property skin is read only"

My Button declaration :

<s:Button id="state"开发者_StackOverflow社区
    label="State View" 
    skin="CustomButtonSkin"            
    click="state_clickHandler(event)" />

This is inside a Component and CustomButtonSkin.mxml is in the same directory ...

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You should set the skinClass property, not skin