public function getFieldValue(string $field_name)
    {
        if (false === $this->validated) {
            $this->validate();
        }
        foreach ($this->validators as $validator) {
            if ($validator->getFieldName() == $field_name) {
                return $validator->getValue();
            }
        }
        throw new InvalidInputException('A field called ' . $field_name . ' was not defined.');
    }