SpawnPlacementRegisterEvent REPLACE operation

I'm trying to change the base rules for vanilla mob spawning. Here my code, what's wrong?

@Mod.EventBusSubscriber(modid = FractumMod.MOD_ID, bus = Mod.EventBusSubscriber.Bus.FORGE )
public class ModSpawn {

    @SubscribeEvent(priority = EventPriority.LOW)
    public static void onSpawnPlacementRegisterEvent(SpawnPlacementRegisterEvent event) throws Exception {
//        event.register(EntityType.DROWNED, SpawnPlacements.Type.ON_GROUND, Heightmap.Types.MOTION_BLOCKING_NO_LEAVES, Placements::checkDrownedSpawnRules, SpawnPlacementRegisterEvent.Operation.REPLACE);
        event.register(EntityType.DROWNED, SpawnPlacements.Type.ON_GROUND, Heightmap.Types.MOTION_BLOCKING_NO_LEAVES, Zombie::checkMonsterSpawnRules, SpawnPlacementRegisterEvent.Operation.REPLACE);
    }
}