mongoose $or with mixed string and _id (ObjectId) not working

I am trying to make a method to fetch a "page" from the document base where the query matches _id or permalink.

The below code example returns a mongoose error:

'Cast to ObjectId failed for value "hello-world" at path "_id" for model "pages"'

Now, obviously the query isn't an ObjectId if the case is 'hello-world' or any other string permalink. So how do I go about using $or in this case, or is there a smarter way to go about it?

/**
 * Describes methods to create, retrieve, update, and delete pages
 * @returns void
 */
function Pages() {
    this.pages = require('../models/pages')
    this.users = require('../models/users')
    require('mongoose').connect(require('../../config/database/mongodb').url)
}

/**
 * Retrieve a page by permalink or id
 * @param {string} pageQuery - id or permalink
 * @callback {function} cFunction
 */
Pages.prototype.getOne = function(pageQuery, cFunction) {
    this.pages.findOne({$or: [{ 'permalink': pageQuery }, { '_id': pageQuery }] })
    .populate('author', 'email')
    .select('title permalink body author')
    .exec(function(error, result) {
        if (error) {
            cFunction(error)
            return
        }
        cFunction(result)
    })
}

via Brian Emilius