Mockito Test fails when service comes to stream.map.collect

Question

This is the method in my service. I get the Exception when .stream.map.collect

@Override
public GameDto save(GameRequestDto gameRequestDto) {

    Optional<Game> gameOptional = gameRepository.findByTitleAndStudio(gameRequestDto.getTitle(),
            gameRequestDto.getStudio());

    if (gameOptional.isPresent()) {
        throw new InvalidRequestException("Game with title: " + gameRequestDto.getTitle() +
                " and studio: " + gameRequestDto.getStudio() + " already exists.");
    }

    Game gameToBeSaved = buildGameToBeSaved(gameRequestDto);
    Game savedGame = gameRepository.save(gameToBeSaved);

    GameDto gameDto = gameMapper.gameToGameDto(savedGame);
    gameDto.setGameGenres(savedGame.getGenres().stream()
            .map(Genre::getGenreName)
            .collect(Collectors.toCollection(HashSet::new)));

    return gameDto;
}

This is my Test

@InjectMocks
GameServiceImpl gameService;

@Mock
GameRepository gameRepository;

@Mock
GenreRepository genreRepository;

@Test
public void saveGameTest() throws Exception {

    Set genres = new HashSet<>();
    genres.add("Role Playing");

    Game savedGame = buildGame(genres);

    Mockito.when(gameRepository.save(any(Game.class))).thenReturn(savedGame);

    gameService.save(buildGameRequestDto(genres));

    Mockito.verify(genreRepository, Mockito.times(4));
    Mockito.verify(gameRepository, Mockito.times(1));
}

And this is the Exception

java.lang.ClassCastException: class java.lang.String cannot be cast to class com.home.myproject.entities.Genre (java.lang.String is in module java.base of loader 'bootstrap'; com.home.myproject.entities.Genre is in unnamed module of loader 'app')