@Override
    public <KR, VR> KStream<KR, VR> flatMap(final KeyValueMapper<? super K, ? super V, ? extends Iterable<? extends KeyValue<? extends KR, ? extends VR>>> mapper) {
        return flatMap(mapper, NamedInternal.empty());
    }
@Test
    public void shouldNotAllowNullMapperOnFlatMap() {
        final NullPointerException exception = assertThrows(
            NullPointerException.class,
            () -> testStream.flatMap(null));
        assertThat(exception.getMessage(), equalTo("mapper can't be null"));
    }