Recieving a nullpointerexception while following the Unity Instantiation tutorial

I am coding this in C#, and I ran into this issue while coding a simple script to shoot a ball from inside of my character. I have figured out it is coming from the AddForce function after casting as a Rigidbody, but can't figure out which value is null.

     using UnityEngine;
 using System.Collections;
 
 public class Shoot : MonoBehaviour {
 
      
     public Rigidbody bulletPrefab;
     public Transform playerCenter;
 
     void Update() {
         if(Input.GetButtonDown("Fire1")) {
             Rigidbody bulletSpawn = Instantiate(bulletPrefab,                            //The prefab spawned
                                       playerCenter.position,                //Gets the position the prefab will go
                                       playerCenter.rotation) as Rigidbody;    //Gets the rotation the prefab will go
 
             bulletSpawn.AddForce(playerCenter.forward * 5000);
         }
 
     }
 
 
     }