assertThrows(
        NullPointerException.class,
        () -> Maps.toMap((Iterable<String>) strings, Functions.constant("foo")));
  }

  public void testToMapWithNullValues() {
    Iterable<String> strings = ImmutableList.of("one", "two", "three");
    assertThrows(NullPointerException.class, () -> Maps.toMap(strings, Functions.constant(null)));
  }