biMap.put("three", null);
    assertThrows(
        IllegalArgumentException.class,
        () -> Maps.asConverter((BiMap<String, Integer>) biMap).convert("three"));
  }

  public void testAsConverter_toString() {
    ImmutableBiMap<String, Integer> biMap =
        ImmutableBiMap.of(
            "one", 1,
            "two", 2);
    Converter<String, Integer> converter = Maps.asConverter(biMap);

    biMap.put("three", null);
    assertThrows(
        IllegalArgumentException.class,
        () -> Maps.asConverter((BiMap<String, Integer>) biMap).convert("three"));
  }

  public void testAsConverter_toString() {
    ImmutableBiMap<String, Integer> biMap =
        ImmutableBiMap.of(
            "one", 1,
            "two", 2);
    Converter<String, Integer> converter = Maps.asConverter(biMap);