RPC call: parameter suddenly null

Hi guys i have this werid bug where a int parameter passed to an RPC suddenly becomes null. The error message looks like this:

 Sending RPC failed because 'UpdateCallbackVariablesUnit' parameter 7 was null
 UnityEngine.NetworkView:RPC(String, NetworkPlayer, Object[])
 MouseFeedbackControl:UpdateTopSelectedUnit(NetworkPlayer, Int32) (at Assets/Scripts/MouseFeedbackControl.cs:307)

The RPC is called by this line in OnGUI() in the client:

 networkView.RPC ("UpdateTopSelectedUnit", RPCMode.Server, Network.player, 0);

And here is the code containing the error. The function is supposed to sync the client to the server (authoritative server):

 [RPC] public void UpdateTopSelectedUnit(NetworkPlayer player, int index) 
     {
     
         int spellcasttype = 0;
         int spellID = 0;
         int buildID = 0;
         string spellname = null;
         string buildname = null;
         
         // PLAYER 1
         if(player.ToString() == "1")
         {
             
             if(selectedUnits1.Count > 0)
             {
                 topSelectedUnit1 = selectedUnits1[0] as GameObject;
                 
                 if(topSelectedUnit1.GetComponent<Unit>().UnitSpells.Count > 0)
                     {
                     spellID = (int)topSelectedUnit1.GetComponent<Unit>().UnitSpells[index];
                     spellname = topSelectedUnit1.GetComponent<Unit>().UnitSpellNames[index] as string;
                     spellcasttype = (int)topSelectedUnit1.GetComponent<Unit>().UnitSpellCastType[index];
                     }
                     
                 if(topSelectedUnit1.GetComponent<Unit>().UnitKnowArray.Count > 0)
                     {
                     buildID = (int)topSelectedUnit1.GetComponent<Unit>().UnitKnowArray[index];
                     buildname = topSelectedUnit1.GetComponent<Unit>().UnitKnowNamesArray[index] as string;
                     }
 //error is in the following line
                 networkView.RPC("UpdateCallbackVariablesUnit", player, player, 
                                      topSelectedUnit1 != null, 
                                      !topSelectedUnit1.GetComponent<Unit>().isStationary,
                                      topSelectedUnit1.GetComponent<Unit>().unitHasAttack,
                                      topSelectedUnit1.GetComponent<Unit>().isBarracks,
                                      spellcasttype,
                                      spellID,
                                         spellname,
                                        buildID,
                                        buildname,
                                       topSelectedUnit1.GetComponent<Unit>().UnitSpells.Count,
                                         topSelectedUnit1.GetComponent<Unit>().UnitKnowArray.Count);
             }
             else 
                 topSelectedUnit1 =  null;
         }
 }

I can't see where the mistake is, please help.