Amplitude dependences of dielectric permittivity ε and dielectric losses tanδ in K 0.88 (NH 4 ) 0.12 H 2 PO 4 mixed crystal near the ferroelectric phase transition temperature T C are investigated. At measurement fields E~ exceeding some critical value, dependence tanδ(E~) can be described within a model that includes the depinning of domain walls from point lattice defects. The activation field is found to grow appreciably when approaching T C from below, due to the presence of nonpolar regions.