setChildIndex of nested mc

programming flash
1

I have an mc on the stage named mcOnStage inside of this mc is another mc named mcInside. I have an mc in the library with a linkage class name of TestMc. I want to instantiate TestMc and then use setChildIndex to put mcInside on top of testMc. This doesn’t work, I get Error #2025: The supplied DisplayObject must be a child of the caller.

    public class Test extends MovieClip {
        
        private var testmc:TestMc;
        public function Test() {

            testmc = new TestMc;
            addChild(testmc);
    
            this.setChildIndex(mcOnStage.mcInside, this.numChildren - 1);
        }
    }

is there a was to achieve this? what am i doing wrong.

help is much appreciated.

Thank you