invalid asset Id if player is connected

I'm trying to implement a small code change to the following project:

https://github.com/michaelmanus/Unet-Client-Server-Example

and in MyServer.cs I'm trying to add:

 [Serializable]
 public class World
 {
     public string StageName;
     public string Message;
     public List<Chests> Chests;
 }

 [Serializable]
 public class Chests
 {
     public string name;
     public int id;
     public string pos;
 }

 public class MyServer : MonoBehaviour
 {

     void Awake()
     {
         Application.runInBackground = true;
         NetworkServer.RegisterHandler(MsgType.Connect, OnConnect);
         NetworkServer.RegisterHandler(MsgType.AddPlayer, OnAddPlayer);
         NetworkServer.RegisterHandler(MsgType.Disconnect, OnPlayerDisconnect);
         NetworkServer.Listen(7777);
         PopulateServerEntities();

         GameObject chest = Instantiate(Resources.Load("Chest"), new Vector3(5,5,5), Quaternion.identity) as GameObject;
         chest.AddComponent<NetworkIdentity>();
         NetworkServer.Spawn(chest);
     }

everything is fine but if the player is also connected I get:

 "OnObjSpawn netId: 2 has invalid asset Id
 UnityEngine.Networking.NetworkIdentity:UNetStaticUpdate()"

any thoughts?

also need to mention that I have

 public enum PrefabType
 {
     Player,
     Npc,
     Chest
 }

set in GlobalAssets.cs

and I have a Chest prefab in my Resources folder