Follow script Error NAN?

Error:

transform.position assign attempt for 'pan' is not valid. Input position is { NaN, -4.357757, 3.197214 }. UnityEngine.Transform:set_position(Vector3) keyfollower:Update() (at Assets/keyfollower.js:18)

Code:

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