public GameObject lookAt;

private bool smooth = true;
private float smoothSpeed = 0.07f;
private Vector3 offset = new Vector3 (0f, 3.5f, -6.5f);

void FixedUpdate()
{
    // THIS LINE CAUSES THE ERROR.
    Vector3 desiredPosition = lookAt.transform.position + offset;

    if(smooth)
    {
        transform.position = Vector3.Lerp(transform.position, desiredPosition, smoothSpeed);
    }
    else
    {
        transform.position = desiredPosition;
    }
}