CharacterController 新手求教

private CharacterController charController;
private void FixedUpdate()
{
charController = GetComponent<CharacterController>();
运行的时候下面这里报错
var flags = charController.Move(moveDirection*Time.deltaTime);//对对象在场景中进行移动
错误提示NullReferenceException: Object reference not set to an instance of an objectI