@Override
    public <KR> KGroupedStream<KR, V> groupBy(final KeyValueMapper<? super K, ? super V, KR> keySelector) {
        return groupBy(keySelector, Grouped.with(null, valueSerde));
    }
@Test
    public void shouldNotAllowNullSelectorOnGroupByWithGrouped() {
        final NullPointerException exception = assertThrows(
            NullPointerException.class,
            () -> testStream.groupBy(null, Grouped.as("name")));
        assertThat(exception.getMessage(), equalTo("keySelector can't be null"));
    }