NullReferenceException when trying to use NetworkManager.SpawnWithClientAuthority

So I want to create a 2d multiplayer game, and I have a gameobject as a player prefab that spawns the actual player when it spawns, and when I try it out with 2 players (1 build and 1 editor), the second player doesn't spawn on the server, and the client (editor) shows this error:

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.Networking.NetworkServer.SpawnWithClientAuthority (UnityEngine.GameObject obj, UnityEngine.Networking.NetworkConnection conn) (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkServer.cs:1565)
Connection.CmdSpawn () (at Assets/Scripts/Connection.cs:20)
Connection.Start () (at Assets/Scripts/Connection.cs:10)

The code:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEngine.Networking;
5. public class Connection : NetworkBehaviour {
6.     public GameObject pobjPrefab;
7.     // Use this for initialization
8.     void Start () {
9.         if (!isLocalPlayer) { return; }
10.         CmdSpawn();
11.     }
12.  
13.     // Update is called once per frame
14.     void Update () {
15.      
16.     }
17.     void CmdSpawn()
18.     {
19.         GameObject pobj = (GameObject)Instantiate(pobjPrefab, transform.position, transform.rotation);
20.         NetworkServer.SpawnWithClientAuthority(pobj, connectionToClient);
21.     }
22. }
23.  

Can someone explain why isn't this working for me? Thanks!