X and Y coords of a movie clip declared in a variable

programming flash
1

How come this:

function drop (thumbClip, xCoord, yCoord) {
	thumbClip._x = xCoord;
	thumbClip._y = yCoord;
	trace (thumbClip);
	trace (xCoord);
	trace(yCoord);
}

receives the variables (specified in a later function) and traces them all right, but refuses to deposit the variable thumbClip in the x and y coordinates being specified?

I replaced thumbClip with a specific movie clip, and the fucntion worked perfectly. Does this mean Flash cannot use variables to set the x and y coords to?

If not, what’s the workaround?