Linking a symbole with a class in a namespace

1

hi there!
I have a fla in wich i have a Sprite linked with a class like:

package fr.tiscar.preloader{
    //import flash.display.Sprite;
    import flash.display.*;
    import flash.events.Event;
    import flash.utils.setTimeout;
    
    public class PreloaderAnimation extends Sprite{
        public var _mask:MovieClip;
.....

the link is written that way:
Class: fr.tiscar.preloader.PreloaderAnimation
Base Class (i guess in english) : flash.display.MovieClip

As you see, in my class i have a variable _mask wich is actually an instance of a simple simple (Sprite).
Whenever i compile my fla it says (in french):

ReferenceError: Error #1056: Impossible de créer la propriété _fond sur fr.tiscar.preloader.PreloaderAnimation.
    at flash.display::Sprite/flash.display:Sprite::constructChildren()
    at flash.display::Sprite$iinit()
    at fr.tiscar.preloader::PreloaderAnimation$iinit()
    at Loader_fla::MainTimeline/Loader_fla::frame1()

BUT, if i remove all the namespace declaration stuff, it compiles well…I really don’t understand.
thanks guys