How to set a Bool value of an NSManagedObject in swift 3/ iOS 10

I am upgrading my app to iOS 10 / swift 3

So constructing an

NSManagedObject

let writerDictionary = [
...: ...,
...: ...,
...:...
]
let newWriter = Writer(dictionary: wirterDictionary, context: shared_context)

isFavorite is an @

NSManaged

newWriter.isFavorite = true

right after constructing the

newWriter

This line crashes with Uncaught Exception "libc++abi.dylib: terminating with uncaught exception of type NSException" in iOS 10

I also tried:

newWriter.setValue(true, forKey: "isFavorite")

It doesn't crash - but the bool value doesn't change?

Previously there was no problem whatsoever with iOS 9

This is the Writer sublclass:

import UIKit
import CoreData
@objc(Writer)
class Writer: NSManagedObject {
   
    /
    @NSManaged var id: NSNumber
    @NSManaged var name: String
    @NSManaged var picture: String?
    @NSManaged var publicationID: NSNumber
    @NSManaged var language: String
    @NSManaged var country: String
    @NSManaged var lastArticle: Date
   
    @NSManaged var isFavorite: Bool           /
   
    /
    @NSManaged var hasNewArticles: Bool
    /
    @NSManaged var publication: Publication
    @NSManaged var articles: [Article]
   
    var writerImage: UIImage? {
        get {
            return ImageCache.sharedCache.imageWithIdentifier("writers-" + String(id))
        }
        set {
            ImageCache.sharedCache.storeImage(newValue, withIdentifier: "writers-" + String(id))
        }
    }
   
    override init(entity: NSEntityDescription, insertInto context: NSManagedObjectContext?) {
        super.init(entity: entity, insertInto: context)
    }
   
    init(dictionary: [String : AnyObject], context: NSManagedObjectContext) {
        let entity = NSEntityDescription.entity(forEntityName: "Writer", in: context)!
        super.init(entity: entity, insertInto: context)
       
        id = ...
        name = ...
        picture = ...
        publicationID = ...
        language = ...
        country = ...
        lastArticle = ...
    }
   
    override func prepareForDeletion() {
        if let _ = writerImage {
            writerImage = nil
        }
    }
}