获取到刚体，取不到速度，怎么回事？

demo Space Shooter
void Start()
{
Rigidbody rb = GetComponent<Rigidbody>();
Vector3 vs = GetComponent<Rigidbody>().velocity;
// 放在这里就不行，敌机不下来
currentSpeed = GetComponent<Rigidbody>().velocity.z;
StartCoroutine(Evade());
}