Gameobject wont move through a function to a ClientRPC

hey guys ive got a networking problem, so ive got this functing that handles muzzleflash in my networked game when it calls it works like this [Command] void CmdOnShoot (GameObject Weapon) {

         RpcDoShootEffect (Weapon);
     }
     //is called on all clients when we need to do a shoot effect
     [ClientRpc]
     void RpcDoShootEffect (GameObject _Weapon)
     {
 //do shoot effect
 }

so when this is called it is suppost to do the effect on all clients, but the problem I get is when the command is called weapon is difined as the weapon you are holding, but when you get to the RPC _weapon is null any help would be appreciated thanks