How to query MongoDb with a different field than _id?

Question

I'd like to retrieve an object from MongoDB other than its _id. I'm using it with react, axios and express. When I try this, I just get an error Error: Request failed with status code 500. How could I fix this? This is my code:

Express:

router.get("/page/:slug", (req, res, next) => {
    Page.findOne({ slug: req.params.slug })
        .then(foundPage => res.status(200).json(foundPage))
        .catch(err => next(err))
})

Axios get:

pageSlug(slug) {
    return axios.get(`http://localhost:5005/api/pages/page/${slug}`)
}

And JSX:

const { slug } = useParams()
const [page, setPage] = useState()

useEffect(() => {
    pageSlug(slug)
        .then(res => {
            setPage(res.data)
        })
        .catch(err => console.log(err))
}, [])

Thanks for your help!

Edit: this is the error I'm getting in the terminal:

ERROR GET /api/pages/page/about CastError: Cast to ObjectId failed for value "about" (type string) at path "_id" for model "Page"
    at model.Query.exec (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/query.js:4641:21)
    at model.Query.Query.then (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/query.js:4740:15)
    at /Users/user/Documents/Github/blog-new/routes/pages.js:17:10
    at Layer.handle [as handle_request] (/Users/user/Documents/Github/blog-new/node_modules/express/lib/router/layer.js:95:5)
    at next (/Users/user/Documents/Github/blog-new/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/Users/user/Documents/Github/blog-new/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/Users/user/Documents/Github/blog-new/node_modules/express/lib/router/layer.js:95:5)
    at /Users/user/Documents/Github/blog-new/node_modules/express/lib/router/index.js:281:22
    at param (/Users/user/Documents/Github/blog-new/node_modules/express/lib/router/index.js:360:14)
    at param (/Users/user/Documents/Github/blog-new/node_modules/express/lib/router/index.js:371:14) {
  messageFormat: undefined,
  stringValue: '"about"',
  kind: 'ObjectId',
  value: 'about',
  path: '_id',
  reason: BSONTypeError: Argument passed in must be a string of 12 bytes or a string of 24 hex characters
      at new BSONTypeError (/Users/user/Documents/Github/blog-new/node_modules/bson/lib/error.js:41:28)
      at new ObjectId (/Users/user/Documents/Github/blog-new/node_modules/bson/lib/objectid.js:65:23)
      at castObjectId (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/cast/objectid.js:24:12)
      at ObjectId.cast (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/schema/objectid.js:247:12)
      at ObjectId.SchemaType.applySetters (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/schematype.js:1180:12)
      at ObjectId.SchemaType._castForQuery (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/schematype.js:1614:15)
      at ObjectId.SchemaType.castForQuery (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/schematype.js:1604:15)
      at ObjectId.SchemaType.castForQueryWrapper (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/schematype.js:1581:20)
      at cast (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/cast.js:344:32)
      at model.Query.Query.cast (/Users/user/Documents/Github/blog-new/node_modules/mongoose/lib/query.js:5076:12),
  valueType: 'string'
}

Edit 2: this is the model:

const pageSchema = new Schema(
    {
        ...other stuff
        slug: {
            type: String,
            unique: true,
        },
    },
)