public Integer apply(String input) {
                    return input.length();
                  }
                });
    assertEquals(expected, index);
  }

  public void testIndex_nullKey() {
    assertThrows(
        NullPointerException.class,
        () -> {
          ImmutableListMultimap<Object, Integer> unused =
              fluent(1, 2, 3).index(Functions.constant(null));
        });

    assertThrows(
        NullPointerException.class,
        () -> Multimaps.index((List<Integer>) values, Functions.identity()));
  }

  public void testIndex_nullKey() {
    List<Integer> values = Arrays.asList(1, 2);
    assertThrows(
        NullPointerException.class, () -> Multimaps.index(values, Functions.constant(null)));
  }

  @GwtIncompatible // untested