Help with my Arrow Fire Script

Hi, im new to c# programing in unity and I have been trying to create a primative arrow fire script but no matter how hard I try I keep get this error after shooting an arrow:

NullReferenceException: Object reference not set to an instance of an object Arrow_Fire.Update () (at Assets/Scripts/Arrow_Fire.cs:15)

Here is the code too so if you have any suggestions it would help greatly (:

using UnityEngine; using System.Collections;

public class Arrow_Fire : MonoBehaviour {

 public Rigidbody arrow;
 public int speed = 20;
 
 void Update () {

     if (Input.GetButtonDown("Fire1")) {
     
         Rigidbody arrowClone = Instantiate(arrow, transform.position, transform.rotation) as Rigidbody;

         arrowClone.velocity = transform.TransformDirection(new Vector3(0, 0, speed));
     }
 }

}