[Tweener] The property '_scale' ....?

[Mirko De Nicolo`]

Ciao Guys,

Could you explain  me why this doesn't work
Tweener.addTween(Mc, {alpha:0.5, _scale:0.5, time:1,  
transition:"easeInOutQuart"});

Tweener] Error: The property '_scale' doesn't seem to be a normal  
object property of [object EnginAnimationSpin] or a registered special  
property

I know i can use scaleX and scaleY but i was expecting that _scale  
would have worked.
I have imported import caurina.transitions.Tweener; ,
Do i need to import anything else?
what I am missing?

Thanks