RealmSwift.Object with numeric optional primary keys can't be added with update=true

[mrackwitz]

Goals

Add an object of a class defined from Swift with a numeric optional primary key to a Realm.

Expected Results

Successfully creating the object.

Actual Results

** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ valueForUndefinedKey:]: this class is not key value coding-compliant for the key (null).'

Code Sample

class Post: Object {
    let postId = RealmOptional<Int>()
    dynamic var postImage : PostImage?
    dynamic var postUser: User?

    override class func primaryKey() -> String? {
        return "postId"
    }
 }

class PostTest : XCTestCase {
   func testExample() {
        let realm = try! Realm()
        realm.beginWrite()
        let post = Post()
        post.postId.value = json["image"]["image_id"].intValue
        realm.add(post, update: true)
        try! realm.commitWrite()
   }
}