Problems with fadein fadeout code

1

This is a code for fading out an object.

function fadeOutObject(target) {
        onEnterFrame = function () {
        target._alpha -= 2;
        if (target._alpha < 0) {
            target._alpha = 0;
            delete onEnterFrame;
        }
    };
}

Now, let’s say I have a movie clip with the instance name of fadetest. If I write:

fadeOutObject(fadetest);

or

fadeOutObject(_root.fadetest)

it doesn’t work… (I’m going crazy with this)…
but if I write:

fadeOutObject(_root);

, then it works… Could somebody explain this to me?