inference variable T has incompatible bounds collectors.tolist in Function.identity

Question

@Override
Public List<TranslationKey> getKeys(){
    return Stream.of(Arrays.stream(Message.values)), Arrays.stream(TranslationKeys.values()))
        .flatMap(Function.identity())
        .collect(Collectors.toList());
}

getting error as inference variable T has incompatible bounds what could be the issue, guide us. thanks