NullReferenceException error

Getting a NullReferenceException error (Object reference not set to an instance of an object) at the line that starts “transform.rotation”.

var targetTag:String;
var speed:float;
var acceleration:float;
var lifeDuration:float = 3.0;
var maxRotation:float;  

function Update()
{
	if(lifeDuration <= 0)
		Destroy(gameObject);
	else
	{
		var targets: GameObject[] = GameObject.FindGameObjectsWithTag("Enemy");
	var closest: GameObject;
		var closestDist = Mathf.Infinity;

		for(Target in targets)
		{
			var dist = (transform.position - Target.transform.position).sqrMagnitude;

			if(dist < closestDist)
			{
				closestDist = dist;
				closest = Target;
			}
		}
		//transform.LookAt(closest.transform);

		transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.LookRotation(closest.transform.position - transform.position), maxRotation * Time.deltaTime);

		speed += Time.deltaTime * acceleration;
		transform.position += transform.forward * speed * Time.deltaTime;		

		lifeDuration -= Time.deltaTime;
	}
}

I don’t know if I am being blind or not but I can’t see where you have defined closest?