TypeError: Error #2007: AS3

Question

I have trouble with my ActionScript 3 code, it gives me this error :

TypeError: Error #2007: Parameter hitTestObject must be non-null. at flash.display::DisplayObject/_hitTest() at flash.display::DisplayObject/hitTestObject() at MethodInfo-8()

This is the code:

//Kills you if you touch an Enemy
addEventListener(Event.ENTER_FRAME, checkCollision);
function checkCollision(e:Event):void
{
    if (square.hitTestObject(newEnemy)){
        //removeEventListener(Event.ENTER_FRAME, checkCollision);
        removeEventListener(Event.ENTER_FRAME,update);
        stage.removeEventListener(KeyboardEvent.KEY_DOWN,jump);
        removeChild(square);
        init();
    }
}

Answer 1

It seems the newEnemy variable is not defined.