public Call<Void> postAsyncReserveDevice(@NonNull ReservedWorkerData data, @NonNull String token) {

        return apiService.postReservedWorker(
                data,
                ApiPrefs.TOKEN_PREFIX + token);
}

@Test
public void test() {
//...
when(apiService.postAsyncReserveDevice(
                eq(data), any(String.class)))
                .thenReturn(Calls.response(Any() as Void))
//...
}

java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.Void