The thing i want to instantiate is null?

Code is supposed to summon a projectile being shot form 1 player to all clients. While the host can summon the projectile for all clients, the clients can't summon, and get the error: ArgumentException: The thing you want to instantiate is null.

     using UnityEngine;
     using System;
     using UnityEngine.Networking;

     public class CharacterController : NetworkBehaviour
     {
     public float fireballSpeed = 6;
     public GameObject fireballPrefab;
     public Transform fireballSpawn;

    void FixedUpdate()
     {
         if (!isLocalPlayer)
         {
             return;
         }
         Attack();
     }
 
     void Attack()
     {
         if (Input.GetButtonDown("Fire1"))
         {
                 CmdFireProjectile(fireballPrefab, fireballSpeed,     fireballSpawn.position,fireballSpawn.rotation);
         }
     }
     [Command]
     void CmdFireProjectile(GameObject projectilePreFab, float projectileSpeed, Vector3 projectileSpawnPosition, Quaternion projectileSpawnRotation)
     {
         GameObject projectile = (GameObject)Instantiate(projectilePreFab, projectileSpawnPosition, projectileSpawnRotation);
 
         projectile.GetComponent<Rigidbody>().velocity = projectile.transform.forward * projectileSpeed;
         NetworkServer.Spawn(projectile);
 
         Destroy(projectile, 2);
     }
   }
 }