Keep getting BCE0017 error

i keep getting the error Assets/Twin_Cannon.js(30,63): BCE0017: The best overload for the method ‘UnityEngine.Quaternion.Lerp(UnityEngine.Quaternion, UnityEngine.Quaternion, float)’ is not compatible with the argument list ‘(UnityEngine.Quaternion, System.Type, float)’. and i dont know why or how to fix, please help

#pragma strict

var myprojectile : GameObject;
var reloadTime : float = 1f;
var turnSpeed : float = 5f;
var firePauseTime : float = .25f;
var muzzleEffect : GameObject;
var errorAmount : float = .001;
var myTarget : Transform;
var muzzlePosition : Transform[];
var turrentBall : Transform;

private var nextFireTime : float;
private var nextMoveTime :float;
private var desiredRotation = Quaternion;
private var aimError : float;

function Start () 


function Update () 
		if(Time.time >= nextMoveTime)
			turrentBall.rotation = Quaternion.Lerp(turrentBall.rotation,desiredRotation,Time.deltaTime*turnSpeed);

Line 15, you declared desiredRotation as being the Quaternion type, not as being a variable of type Quaternion


private var desiredRotation = Quaternion;

Should be

private var desiredRotation : Quaternion;