public function get404Properties()
    {
        $result = [];
        foreach ($this->result as $propertyName => $stuff) {
            if (404 === $stuff[0]) {
                $result[] = $propertyName;
            }
        }
        // If there's nothing in this list, we're adding one fictional item.
        if (!$result) {
            $result[] = '{http://sabredav.org/ns}idk';
        }

        return $result;
    }