`req.cookies` is usually assigned a plain object where middleware populates the cookie name and value on it. However, some middlewares opt to store the cookie jar in `req.cookies` and then inject each cookie property directly on `req.headers.cookies`. This is the case of the [cookies middleware](https://github.com/expressjs/cookies/blob/77d7b28811600ae87f719a4e8b236cb3abe6ebeb/lib/cookies.js#L136) in expressjs. However, it does not include a custom `toString()` method to expose the cookie jar as an object, which would result in an unsupported value.