[TargetRPC] is causing a NullReferenceException... HELP!

So I updated to the 5.4 beta. I want to test the new [TargetRPC] with a simple "Hello world!" script:

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3. using UnityEngine.Networking;
4.  
5. public class RampUp : NetworkBehaviour {
6.  
7.     void OnTriggerEnter(Collider target){
8.         if (target.tag == "Hand_NoTeam" || target.tag == "Hand_Green" || target.tag == "Hand_Orange" || target.tag == "Hand_Blue") {          
9.             CmdRampUp();      
10.         }
11.     }
12.  
13.     [TargetRpc]
14.     public void TargetRampUp(NetworkConnection target){
15.         Debug.Log ("Cheeseburger!");
16.     }
17.  
18.     [Command]
19.     void CmdRampUp (){
20.         TargetRampUp (connectionToClient);
21.     }
22. }

The result:

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.Networking.NetworkBehaviour.SendTargetRPCInternal (UnityEngine.Networking.NetworkConnection conn, UnityEngine.Networking.NetworkWriter writer, Int32 channelId, System.String rpcName) (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkBehaviour.cs:122)
RampUp.CallTargetRampUp (UnityEngine.Networking.NetworkConnection target)
RampUp.CmdRampUp () (at Assets/Scripts/Vibration/RampUp.cs:20)
RampUp.CallCmdRampUp ()
RampUp.OnTriggerEnter (UnityEngine.Collider target) (at Assets/Scripts/Vibration/RampUp.cs:9)


I'm really new to all the netcode stuff, can somebody explain that NullRefeferenceException to me?