ArgumentException: The Object you want to instantiate is null.

I am trying to pass the WeaponGrap$$anonymous$$c through the Instantiate method in a ClientRpc Call. However I need first to declare it to the server. So the weapon grap$$anonymous$$c need to be passed over through two methods. When I try t$$anonymous$$s I get the following error:

ArgumentException: The Object you want to instantiate is null.

Here is my code:

  public void BuyAk(){
  CmdSpawnThatS$$anonymous$$tWeapon(0, _weapon.Ak.WeaponGfx);
  }
  [Command]
      void CmdSpawnThatS$$anonymous$$tWeapon(int index, GameObject gfx1)
      {
          RpcSpawnThatS$$anonymous$$tWeapon(index,gfx1);
      }
  [ClientRpc]
      void RpcSpawnThatS$$anonymous$$tWeapon(int index, GameObject gfx)
      {
          _weaponIns[index] = Instantiate(gfx, _weapon.WeaponHolder.position, _weapon.WeaponHolder.rotation);
  }

For debbuging: If I replace gfx normally with _weapon.Ak.WeaponGfx the code work fine and no error is thrown. However I am using other weapons and I want to pass other grap$$anonymous$$cs. So how do I proceed? Thank you for helping me.