Dynamically place instances of MovieClip on stage (AS3)

Question

I'm trying to dynamically place instances of MovieClip on the stage. Receiving an error:

TypeError: Error #2007: Parameter child must be non-null.
    at flash.display::DisplayObjectContainer/addChild()

ActionScript:

         // properties in class ----------
         var circle_ary:Array = new Array;  
         var circ_num:int;//number of circles on the stage

         // dynamically place MovieClip instances on the stage ----------
        // inside Constructor function of class
        var circ_num:int=20;//number of circles on the stage

        for(var i=0; i<circ_num; i++) {
            circle_ary[i] = new Circle(); //linkage in the library
            addChild(Circle[i]);
        }

Answer 1

It should be

addChild(circle_ary[i]);