# Exchange rate target zone 45 degree line in the free float case - Krugman Target Zones

As far as i understand in the Krugman target zone model the following equation describes the behaviour of the exchange rate within and outside a target zone $$s_t = f_t + \alpha E_t(ds_t/dt)$$. But if the fundamental $$f_t$$ follows a random walk why is $$s_t = f_t$$ (the 45 degree line on the S-curve) a solution to the equation in the free float case?