public function normalize($value)
    {
        if (is_null($value) || $value === '') {
            return;
        }
        if (is_bool($value)) {
            return $value;
        }

        if ($value === $this->valueTrue) {
            return true;
        }

        if ($value === $this->valueFalse) {
            return false;
        }

        if (is_numeric($value)) {
            return (bool) $value;
        }

        throw new ValidationException([$this->name => 'Must be a boolean value']);
    }