The PADME experiment at the DAΦ NE Beam-Test Facility (BTF) aims at searching for invisible decays of the dark photon by measuring the final state missing mass in the process e + e − → γ + A ′ , with A ′ undetected. The measurement requires the determination of the 4-momentum of the recoil photon, performed using a homogeneous, highly segmented BGO crystals calorimeter . We report the results of the test of a 5×5 crystals prototype performed with an electron beam at the BTF in July 2016.