Flex2 and namespaces extending components

I took 2 hours to find a solution for that and I decided to blog it..
I was creating my new Flex2 component extending the UIComponent class and I need to define new custom styles for my own component…
Ok, i opened the “create and extend components” PDF guide and found that I need to put this code to accomplish it:
private static function classConstruct():Boolean
if (!StyleManager.getStyleDeclaration("ResizeManager"))
var newStyleDeclaration:CSSStyleDeclaration = new CSSStyleDeclaration();
newStyleDeclaration.setStyle("boxSize", 4);
StyleManager.setStyleDeclaration("ResizeManager", newStyleDeclaration, true);
return true;

That’s all, but at this time the compiler die with that error message “1000: Riferimento ambiguo a setStyle”, which in english should be: “1000: Ambiguous reference to setStyle“.
I google for some time but without finding a solution, then I discovered the problem. I was using in my component the mx_internal namespace, that was the problem. Once changed namespace everything was ok.