why do i get a not valid error?

why do i get this error?

transform.position assign attempt for 'Main Camera' is not valid. Input position is { 30.810999, 0.308909, NaN }.
UnityEngine.Transform:set_position(Vector3)
cmaerafolloebird:Update() (at Assets/effects 1/cmaerafolloebird.js:14)

here is my code JavaScript

 #pragma strict
var target : Transform;
var smoothTime = 0.3;
private var Transform : Transform;
private var velocity : Vector3;
var offsetAmountx : float = 10.00;
var offsetAmountz : float = 10.00;
function Start () {
    Transform = transform;
}
function Update () {
         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);
}

I can answer this in two ways:

a) The vector that you assign to the camera position has one element (z) set to NaN. NaN is “not a number” so the location of the camera is not defined. You’re not allowed to do this since so much of the runtime of Unity needs to know the position of the camera in the world. The NaN is computed in your code. Your variable velocity is never given a value, so when you read from it you get completely bogus values.

b) The string Transform starts with a capital T. By convention this means it’s a class. You’ll see in your code that you have a private variable called Transform which is of type Transform. This looks somewhat dodgy to me. Rename your variable Transform to be something like myTransform. Or, in fact, do away with your variable completely. In Javascript code every game object has a variable called transform, which is the transform that positions the game in the world.