@Override
    public KStream<K, V> merge(final KStream<K, V> stream) {
        return merge(stream, NamedInternal.empty());
    }
@Test
    public void shouldNotAllowNullKStreamOnMerge() {
        final NullPointerException exception = assertThrows(
            NullPointerException.class,
            () -> testStream.merge(null));
        assertThat(exception.getMessage(), equalTo("stream can't be null"));
    }