Type Error #1123: what and why

programming flash
1

so, the fighting game class creates an instance of fighter, then adds a child of “dude” to fighter. I want to animate dude, so I tried doing this:

for (var i:uint = 0; i < fighter.numChildren; i++)
			var mychild = fighter.getChildAt(i);
			fighter.addChild(dude);

thats in the constructor.

fighter.(MyChild as MovieClip.gotoAndPlay("walkingright");

that is within the fightinggame.as. Now, the game runs, but when I press right ( the keys that cause that movieclip to animate), it throws a -

TypeError: Error #1123: Filter operator not supported on type Fighter.
at FightingGame/Update()[C:\Users\Olsen\Documents\FightingGame\FightingGame.as:399]
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()

Also the movieclip does not show up, and does not animate. What is this, I can find little information about it, and how do i fix it? the area where it throws the error is the gotoandplay line.