Simple Script Error?

why do i get this error?

 transform.position assign attempt for 'key' is not valid. Input position is { NaN, -0.337238, 3.047216 }.
 UnityEngine.Transform:set_position(Vector3)
 follow:Update() (at Assets/Scripts/follow.js:16)

and here is the code:

 #pragma strict
 var target : Transform;
 var smoothTime = 0.3;
 private var Transform : Transform;
 private var velocity : Vector3;
 var offsetAmountx : float = 10.00;
 var offsetAmounty : float = 10.00;
 var offsetAmountz : float = 10.00;
 
 function Start () {
     Transform = transform;
 }
 function Update () {
             Transform.position.y = Mathf.SmoothDamp( Transform.position.y, 
        target.position.y +offsetAmounty,velocity.y, smoothTime);
             Transform.position.x = Mathf.SmoothDamp( Transform.position.x, 
        target.position.x +offsetAmountx,velocity.x, smoothTime);
             Transform.position.z = Mathf.SmoothDamp( Transform.position.z, 
        target.position.z +offsetAmountz,velocity.z, smoothTime);
 }