NullPointerException.class, () -> builder.put(immutableCell('a', 1, (String) null)));
  }

  private static class StringHolder {
    @Nullable String string;
  }

  public void testBuilder_withMutableCell() {
    ImmutableTable.Builder<Character, Integer, String> builder = new ImmutableTable.Builder<>();

    final StringHolder holder = new StringHolder();
    holder.string = "foo";