Add child creates little white box

I have this code + background color set to grey.
The weirdest thing is that addChild(something); seems to create a white 46x46 pixel white box on stage.
If I take addChild away, white box goes away as well…

WHAT IS GOING ON!?

Timeline script:

var something:Main = new Main();
addChild (something);

Main.as:

package
{
   import flash.display.MovieClip;
    
    public class Main extends MovieClip
    {
        public function Main()
        {
            trace(" Main class instantiated");
            //class content begins..
        }
    }
}