.put('a', 1, "foo")
            .put('a', 1, "bar");
    assertThrows(IllegalArgumentException.class, () -> builder.build());
  }

  public void testBuilder_noNulls() {
    ImmutableTable.Builder<Character, Integer, String> builder = new ImmutableTable.Builder<>();
    assertThrows(NullPointerException.class, () -> builder.put(null, 1, "foo"));