帮忙看看这段代码， 为啥不能实现遮罩效果

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代码:

this.createEmptyMovieClip("content_mc",this.getNextHighestDepth());
            with("content_mc") {
                //_alpha=30;
                _x = 0;
                _y = 0;
                _width = 10;
                _height = 10;
                //_xscale = 10;
                //_yscale = 10;
                loadMovie ( "xxx.jpg" );
            }
//this.attachMovie ("content_mc","contents_name",this.getNextHighestDepth());
trace(content_mc._width);
this.createEmptyMovieClip("mask_mc",this.getNextHighestDepth());
            with("mask_mc") {
                //_alpha=30;
                _x = 0;
                _y = 0;
                _width = 10;
                _height = 10;
                //_xscale = 10;
                //_yscale = 10;
                //loadMovie ( "kejian.jpg" );
            }
//this.attachMovie ("mask_mc","mask_name",this.getNextHighestDepth());
trace(mask_mc._width);
content_mc.setMask(mask_mc);