Prefab not spawning on client correctly

I'm making a multiplayer fps, and I'm trying to add a gun to the player as soon as they spawn. On the host it works great, but on the client it says 'ArgumentException: The thing you want to instantiate is null'. I'm running one instance on the editor and one in a build, if that helps. The weapons all have NetworkIdentities. Weapon spawning code:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.Networking;
 
 public class WeaponryHandler : NetworkBehaviour {
     //Public vars
     public GameObject Gimbal;
     public GameObject StartingWeapon;
     //Private vars
     private GameObject CurrentWeapon;
 
     public override void OnStartLocalPlayer ()
     {
         base.OnStartLocalPlayer ();
         Debug.Log (isLocalPlayer);
         Cmd_EquipWeapon (StartingWeapon);
     }
 
     [Command]
     void Cmd_EquipWeapon(GameObject Weapon){//Try read Prefab from file
         Debug.Log ("rekskoped");
         GameObject wep = (GameObject)Instantiate (Weapon, Vector3.zero, Quaternion.identity);
         wep.transform.parent = Gimbal.transform;
         NetworkServer.Spawn (wep);
         Rpc_SyncGun (wep);
     }
 
     [ClientRpc]
     void Rpc_SyncGun(GameObject wep){
         wep.transform.parent = Gimbal.transform;
         wep.transform.localPosition = Vector3.zero;
         wep.transform.localRotation = Quaternion.identity;
     }
 }