fail();
    } catch (IllegalArgumentException expected) {
      assertThat(expected.getMessage()).contains("Multimaps.index");
    }
  }

  /** Null values are not allowed. */
  public void testUniqueIndexNullValue() {
    List<@Nullable String> listWithNull = Lists.newArrayList((String) null);
    try {
      Maps.uniqueIndex((List<String>) listWithNull, Functions.constant(1));
      fail();