Getting this error when using Network.Instantiate

The object is not the main asset. Only root objecs of a prefab may be used for Instantiating UnityEngine.Network:Instantiate(Object, Vector3, Quaternion, Int32) SpawnManager:OnGUI() (at Assets/Script/SpawnManager.cs:45)

Here's my script

//27/01/2015 Mads Winther

        using UnityEngine;
       using System.Collections;

      public class SpawnManager : MonoBehaviour {
 public GameObject RunnerPlayer;
 public GameObject RunnerCamera;
 public GameObject DeathPlayer;
 public string CurTeam = "";
 private bool Dead = true;
 public GameObject SpawnPointRunner;
 public GameObject SpawnPointDeath;
 public int Runners = 0;
 public int Death = 0;
 public int MaxRunners;
 public int MaxDeath ;
 public float CenterW;
 public float CenterH;

 public bool NewRound = false;


 // Use this for initialization
 void Start () {

     CurTeam = "";    
     CenterW = Screen.width / 2 - 150;
     CenterH = Screen.height / 2 - 80;
 }
 
 // Update is called once per frame
 void Update () {
 
 }

 void OnGUI () {

 
                     
             
                     if (CurTeam == "") {
                             GUI.Box (new Rect (CenterW, CenterH, 300, 160), "Select Team");
                             if (GUI.Button (new Rect (CenterW + 5, CenterH + 20, 290, 60), "Runner Team")) {
                                     Network.Instantiate (RunnerPlayer, SpawnPointRunner.transform.position, transform.rotation, 0);
                                     Network.Instantiate (RunnerCamera, SpawnPointRunner.transform.position, transform.rotation, 0);
                                     CurTeam = "Runner";
                                     Runners++;
                             }

                             if (GUI.Button (new Rect (CenterW + 5, CenterH + 90, 290, 60), "Death Team")) {
                                     Network.Instantiate (DeathPlayer, SpawnPointDeath.transform.position, transform.rotation, 0);
                                     CurTeam = "Death";
                                     Death++;
                             }
                     } else
     
     if (NewRound == true) {
                             Respawn ();
                     }
 
             

     }

 void OnDisconnectedFromServer(NetworkDisconnection info) {
     if (Network.isServer)
         Debug.Log("Local server connection disconnected");
     else
         if (info == NetworkDisconnection.LostConnection)
             Debug.Log("Lost connection to the server");
     if(CurTeam == "Runner") {
         Runners--;
     }
     
     if(CurTeam == "Death") {
         Death--;
     }
     else
         Debug.Log("Successfully diconnected from the server");
     if(CurTeam == "Runner") {
         Runners--;
     }
     
     if(CurTeam == "Death") {
         Death--;
     }
 }

 void Respawn() {

     if(CurTeam == "Runner") {
     Network.Instantiate (RunnerPlayer, SpawnPointRunner.transform.position,transform.rotation, 0);
         Network.Instantiate (RunnerCamera, SpawnPointRunner.transform.position,transform.rotation, 0);
         NewRound = false;
         Dead = false;
     }

     if(CurTeam == "Death") {
         Network.Instantiate (DeathPlayer, SpawnPointDeath.transform.position,transform.rotation, 0);
         NewRound = false;
         Dead = false;
     }
 }

}