'None' instance does not serialize well with Salat+Mongo

mongo-entry.json

{
    "_id": {
        "$oid": "4fc5f4bde4b02e0b2013e343"
    },
    "poll": {
        "_id": 7,
        "name": "",
        "created": 1338373308897,
        "question": "Would this be better if you didn't have to click 'confirm your vote' as well as selecting an option?",
        "option_a": "Absolutely, your keen UX sense is unparalleled",
        "option_b": "Of course not, you're an idiot",
        "expires": 1338373609407,
        "short_url": "http://plrz.me/Lp8NnK",
        "user": {
            "_id": 12,
            "email": "user@example.org",
            "name": "Example user",
            "mobile": "",
            "created": 1338373145964,
            "avatarURL": "https://secure.gravatar.com/avatar/47d1af0e885746e39195c8ff3234f47d?s=50&d=https://d37eplffuz3ep4.cloudfront.net/av67.png"
        }
    }
}

ResponsePoll.scala

case class ResponsePoll(
    id: Long, 
    name: String, 
    created: Option[Long],
    question: String,
    option_a: Option[String],
    option_b: Option[String],
    image_a: Option[String],
    image_b: Option[String],
    expires: Option[Long],
    short_url: Option[String],
    allow_anon: Option[Boolean],
    has_voted: Option[Boolean],
    user: Option[ResponseUser], /* owner */
    userId: Option[Long], /* owner ID */
    mongoObjectId: Option[String]
    )

case class MegaPoll(poll: ResponsePoll /*, some other unimportant params */)

stacktrace

[error] application - MongoActor error=[java.lang.IllegalArgumentException: can't serialize class controllers.ResponseUser]
java.lang.IllegalArgumentException: can't serialize class controllers.ResponseUser
	at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:234) ~[mongo-java-driver-2.7.3.jar:na]
	at org.bson.BasicBSONEncoder.putIterable(BasicBSONEncoder.java:259) ~[mongo-java-driver-2.7.3.jar:na]
	at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:198) ~[mongo-java-driver-2.7.3.jar:na]
	at org.bson.BasicBSONEncoder.putIterable(BasicBSONEncoder.java:259) ~[mongo-java-driver-2.7.3.jar:na]
	at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:198) ~[mongo-java-driver-2.7.3.jar:na]
	at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:140) ~[mongo-java-driver-2.7.3.jar:na]
java.lang.IllegalArgumentException: can't serialize class controllers.ResponseUser
	at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:234)
	at org.bson.BasicBSONEncoder.putIterable(BasicBSONEncoder.java:259)
	at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:198)
	at org.bson.BasicBSONEncoder.putIterable(BasicBSONEncoder.java:259)
	at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:198)
	at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:140)
	at org.bson.BasicBSONEncoder._putObjectField(BasicBSONEncoder.java:190)
	at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:140)
	at org.bson.BasicBSONEncoder.putObject(BasicBSONEncoder.java:86)
	at com.mongodb.DefaultDBEncoder.writeObject(DefaultDBEncoder.java:27)
	at com.mongodb.OutMessage.putObject(OutMessage.java:142)
	at com.mongodb.DBApiLayer$MyCollection.update(DBApiLayer.java:346)
	at com.mongodb.DBCollection.update(DBCollection.java:165)
	at com.mongodb.casbah.MongoCollectionBase$class.update(MongoCollection.scala:701)
	at com.mongodb.casbah.MongoCollection.update(MongoCollection.scala:920)
	at com.novus.salat.dao.SalatDAO.update(SalatDAO.scala:383)
	at util.mongo.MegaPoll$.update(Mongo.scala:37)
	at util.mongo.MongoActor$$anonfun$receive$1$$anonfun$apply$2.apply(MongoActor.scala:54)
	at util.mongo.MongoActor$$anonfun$receive$1$$anonfun$apply$2.apply(MongoActor.scala:43)
	at scala.Option.map(Option.scala:133)
	at util.mongo.MongoActor$$anonfun$receive$1.apply(MongoActor.scala:43)
	at util.mongo.MongoActor$$anonfun$receive$1.apply(MongoActor.scala:17)
	at akka.actor.Actor$class.apply(Actor.scala:311)
	at util.mongo.MongoActor.apply(MongoActor.scala:15)
	at akka.actor.ActorCell.invoke(ActorCell.scala:619)
	at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:196)
	at akka.dispatch.Mailbox.run(Mailbox.scala:178)
	at akka.dispatch.ForkJoinExecutorConfigurator$MailboxExecutionTask.exec(AbstractDispatcher.scala:505)
	at akka.jsr166y.ForkJoinTask.doExec(ForkJoinTask.java:259)
	at akka.jsr166y.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:974)
	at akka.jsr166y.ForkJoinPool.runWorker(ForkJoinPool.java:1478)
	at akka.jsr166y.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)