Mongoose primitive set/setter is called twice when running a query

mongoose-primitive-set-test.js

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const schema = new Schema({
    item: {
        type: String,
        set: function(item) {
            console.log('item setter called')
            return normalize(item);
        }
    }
});
const Model = mongoose.model('Object', schema);

function normalize(url) {
    return url.substr(0, url.length - 1);
}

(function main() {
      const item = new Model({item: 'item'});
      item.save();

      Model.findOne({item: 'item'})
        .then(console.log)
        .catch(console.error)
})();

//Output: 
// "item setter called" (save)
// "item setter called"
// "item setter called"
// null

//Expected:
// "item setter called" (save)
// "item setter called" (find)
// [Object]