You are getting the right thing. This is the binding energy formula.
$$E_{binding} = (M_{constituents}-M_{BoundState})c^2$$$$E_{\text{binding}} = (M_{\text{constituents}}-M_{\text{BoundState}})c^2$$
When the constituents come together to form a bound state the total mass is lowered not raised. Binding energy is the energy corresponding to the mass lost by the constituents as a result of them entering the bound state.