I hava a problem i am making a Top Down game and i have a space ship that has a turret on top i have this script to make the turret look at he mouse position

private var worldPos : Vector3;
private var mouseX : int;
private var mouseY : int;
private var cameraDif : int;
var fpc : GameObject;

function Start () {

    //determines how far down the ScreenToWorldPoint is from the camera position
    //it's calculated [height of camera] - [height of pivot point of character]
    //this is to ensure the character only rotates (via LookAt) along rotation.x and doesn't look up or down
    cameraDif = camera.transform.position.y - fpc.transform.position.y;


}

function Update () {

    mouseX = Input.mousePosition.x;
    mouseY = Input.mousePosition.y;


    //this takes your current camera and defines the world position where your mouse cursor is at the height of your character -->translates your onscreen position of mouse into world coordinates
    worldPos = camera.ScreenToWorldPoint(Vector3(mouseX, mouseY, cameraDif));

    fpc.transform.LookAt(worldPos);

}

I attacked this to the camera and the fpc is set as the turret but when i play the game my turret also looks up and down i only want it to look around can someone please help?

Thanks.