assertFailsToDecode(base64(), "A=BCD", "Invalid input length 5");
    assertFailsToDecode(base64(), "?", "Invalid input length 1");
  }

  public void testBase64CannotUpperCase() {
    assertThrows(IllegalStateException.class, () -> base64().upperCase());
  }

  public void testBase64CannotLowerCase() {
    assertThrows(IllegalStateException.class, () -> base64().lowerCase());
  }