Invoking Mongoose document.save() does not appear to save?

I'm attempting to save an object to my database; I know the database is active, because I've read the object in via the same connection being used to update it.

saveAndReturnQuiz = (res, quiz) ->
  quiz.save (err) ->
    # At some point, started seeing empty object for err, rather than null
    if not _.isEmpty err
      switch err?.code
        when 11000
          sendError res, "Quiz title must be unique"
        else
          console.error "Unexpected error on save(): %j", err
          sendError res, extractError err

      return

    console.log "Sending Quiz response: %j", quiz

    res.send quiz

What I'm seeing is that the call to save() is failing, but passing an empty err object.

Here's my schema:

# Defines the schema of data
# Exports three Mongoose Model objects: Question, Round, and Quiz

mongoose = require "mongoose"

Schema = mongoose.Schema

Question = new Schema
  kind:
    type: String
    enum: [ "text" ]
  text:
    type: String
    required: true
  answer:
    type: String
    required: true
  value: Number
  { strict: true }

Round = new Schema
  kind:
    type: String
    required: true
    enum: ["normal", "challenge", "wager"]
  title:
    type: String
    required: true
  questions: [Question]
  { strict: true }

Quiz = new Schema
  title:
    type: String
    required: true
    unique: true
  created:
    type: Date
    default: -> new Date()
  location: String
  rounds: [Round]
  { strict: true }

module.exports =
  Question: mongoose.model('Question', Question)
  Round: mongoose.model('Round', Round)
  Quiz: mongoose.model('Quiz', Quiz)

So perhaps I'm just dealing with these embedded elements incorrectly?

Interestingly, I can add new Quiz objects to the database, just not update the existing ones:

  app.post "/api/quiz",
    (req, res) ->
      quiz = new Quiz req.body
      saveAndReturnQuiz res, quiz

  # Update an existing Quiz
  app.put "/api/quiz/:id",
    (req, res) ->
      Quiz.findById req.params.id,
        handleError res, (quiz) ->
          _.extend quiz, req.body
          console.log "Ready to save: %j", quiz
          saveAndReturnQuiz res, quiz

A further update: when the model has no nested elements, the update appears to work correctly. It is just when there are nested elements (Rounds, and within Rounds, Questions) that things fail.

I suspect there's something I'm missing in setting up my schema, but I'm not sure what to check next.

Thanks in advance for any help!