assertThrows(
        NullPointerException.class,
        () -> builder.putAll(Collections.<String, Integer>singletonMap("one", null)));
  }

  public void testPuttingTheSameKeyTwiceThrowsOnBuild() {
    Builder<String, Integer> builder =
        new Builder<String, Integer>()
            .put("one", 1)
            .put("one", 1); // throwing on this line might be better but it's too late to change