private function checkContent($content)
    {
        if (! is_array($content) and ! is_string($content)) {
            throw new InvalidTypeException(
                'The content must be an array or string value, ' . gettype($content) . ' is given.'
            );
        }

        $this->checkSingleValuedContent($content);

        return is_array($content) ? $content : [$content];
    }