TypeError: Cannot read property 'populate' of undefined

Mongoose перестал обрабатывать ".populate", выдает ошибку: TypeError: Cannot read property 'populate' of undefined.

Package.json

"dependencies": {
"connect-mongodb-session": "^2.2.1",
"express": "^4.16.4",
"express-handlebars": "^3.0.2",
"express-session": "^1.16.1",
"mongoose": "^5.5.3",
"uuid": "^3.3.2"

}

Models/

const itemSchema = new Schema({
  title: String,
  price: Number,
  userId: {
    type: Schema.Types.ObjectId,
    ref: 'User'
  }
})

App.js

router.get('/', async (req, res) => {
  const user = await req.user
  .populate('cart.items.itemId')
  .execPopulate()

  res.render('cart')
})

Подскажите, в чем может быть проблема? Из-за обновления версии mongoose?