How can I differentiate a tampered signed cookie from a `false` JSON cookie?

Forewarning: I am _extremely_ new to Node.js and NPM.

From reading the documentation of `cookieParser`, it appears that cookies that are signed but fail validation are assigned `false` in the `req.signedCookies` object.

> Signed cookies that fail signature validation will have the value `false` instead of the tampered value.

Also, cookies prefixed with `j:` are automatically parsed with `JSON.parse`.

> In addition, this module supports special "JSON cookies". These are cookie where the value is prefixed with `j:`. When these values are encountered, the value will be exposed as the result of `JSON.parse`.

However, the string `'false'` is valid JSON, so it should get parsed to `false`. I also found no option to disable the built-in JSON parser. How can I differentiate a valid signed `j:'false'` cookie from an invalid signed cookie?

If this is not possible, I request that this be relabeled to a bug report or feature request.