ArgumentException: The Object you want to instantiate is null.

I am trying to pass the WeaponGraphic through the Instantiate method in a ClientRpc Call. However I need first to declare it to the server. So the weapon graphic need to be passed over through two methods. When I try this I get the following error:

ArgumentException: The Object you want to instantiate is null.

Here is my code:

  public void BuyAk(){
  CmdSpawnThatShitWeapon(0, _weapon.Ak.WeaponGfx);
  }
  [Command]
      void CmdSpawnThatShitWeapon(int index, GameObject gfx1)
      {
          RpcSpawnThatShitWeapon(index,gfx1);
      }
  [ClientRpc]
      void RpcSpawnThatShitWeapon(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 graphics. So how do I proceed? Thank you for helping me.