Decoder function not being called

Hi, I'm getting crazy with this but for some reason the decoder function seems to never be called. Here is my code:

  router.get('/testqs', function(req, res) {
    const result = qs.parse(req.query, {
      decoder: function (str) {
        let result: any
        const keywords: any = {
          true: true,
          false: false,
          null: null,
          undefined: undefined
        }

        if (str in keywords) {
          result = keywords[str]
        } else {
          const parsed = parseFloat(str)
          if (isNaN(parsed)) {
            result = str
          } else {
            result = parsed
          }
        }

        console.log(str, result)
        return result
      }
    })
    res.json(result)
  })

When I send a request:

GET {{host}}{{api}}/testqs?maintYear=2018&maintMonth=5&fields[]=id&fields[]=regname&fields[]=price&notRenewed=true

I get the following result:

{
  "maintYear": "2018",
  "maintMonth": "5",
  "fields": [
    "id",
    "regname",
    "price"
  ],
  "notRenewed": "true"
}

To summarize, evrrything seems to work properly except that the decoder function seems to never be called (and of course, I don't have any console.log output from inside the function).

I noticed that someone in the following comment (https://github.com/ljharb/qs/issues/91#issuecomment-358708095) that someone is saying that this doesn't seem to work for him but not sure if he doesn't like the result or if he encounters the same problem as I do.

Any idea regarding what could go wrong ?

Thanks