assertThrows(NullPointerException.class, () -> equalsTester.addEqualityGroup((Object) null));
  }

  /** Test equalObjects after adding multiple instances at once with a null */
  public void testAddTwoEqualObjectsAtOnceWithNull() {
    assertThrows(
        NullPointerException.class,
        () -> equalsTester.addEqualityGroup(reference, equalObject1, null));
  }

  /** Test adding null equal object yields error */