@Override
    public <VR> KTable<K, VR> mapValues(final ValueMapper<? super V, ? extends VR> mapper) {
        Objects.requireNonNull(mapper, "mapper can't be null");
        return doMapValues(withKey(mapper), NamedInternal.empty(), null);
    }
@Test
    public void shouldNotAllowNullMapperOnMapValues() {
        assertThrows(NullPointerException.class, () -> table.mapValues((ValueMapper) null));
    }