assertThrows(NullPointerException.class, () -> helper.add(null, "Hello"));
  }

  @GwtIncompatible // Class names are obfuscated in GWT
  public void testToString_addWithNullValue() {
    final String result = MoreObjects.toStringHelper(new TestClass()).add("Hello", null).toString();

    assertEquals("TestClass{Hello=null}", result);
  }