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));
	}
}

}

That probably happens because you didn’t assign an arrow prefab to the script in the properties.