added validation back

      The index of unknown character is `len(charset)`.

    """

    if max_length is not none:

      if (len(string) > self.max_length):

        logger.info(

            "The length of {} is longer than `max_length`. So we return an empty array."

        )

        return np.array([])

      string = self.pad_string(string)  # Padding

    return np.array([

        one_hot_encode(val, self.charset, include_unknown_set=True)

        for val in string