The Resistive Wall Mode (RWM) dispersion relation is analytically derived for a cylindrical plasma surrounded by two walls: the first wall (facing the plasma) is assumed thin (geometrically and magnetically) and resistive, while the second wall of larger radius is ferromagnetic, but nonconductive. The predictions of this simplified model are verified on the JET tokamak, by a comparison with the calculations of a linearized axisymmetric plasma response model. In this case, it is found that, assuming all active and passive conducting structures as short circuited, the presence of iron causes a constant increase of the n = 0 RWM growth rate of around 25s-1, independently of the plasma configuration under study.