可编码NSManagedObject在decodeIfPresent数据类型上失败

当我试图解码这个json时，我得到了这个错误：

[{
    "id": "76f22c25-cee7-4c7a-94fa-1fb85720f580",
    "purchaseDate": "2012-04-05T19:03:43Z",
    "title": "azare",

}, {
    "id": "9b4b9f7d-382f-4555-9eaa-97939b13633f",
    "purchaseDate": "2012-04-05T19:02:46Z",
    "title": "Chocolat",

}, {
    "id": "02a0aa06-2d0c-4ab9-aaaa-af7dee7b4845",
    "purchaseDate": "2012-09-24T17:39:52Z",
    "title": "Some thing",
}]

致命错误：'试试！'表达式意外地引发了一个错误：Swift.DecodingError.typeMismatch（Swift.Double，Swift.DecodingError.Context（codingPath：[Foundation . （_ JSONKey in _12768CA107A31EF2DCE034FD75B541C9）（stringValue：“Index 0”，intValue：Optional（0）），MyApp . Purchase.CodingKeys.purchaseDate]，debugDescription：“预计解码Double但发现字符串/数据 . ”，underlyingError：nil））：file /BuildRoot/Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang- 900.0.69.2/src/swift/stdlib/public/core/ErrorType.swift，第181行

My code:

//Purchase+CoreDataClass.swift
@objc(Purchase)
public class Purchase: NSManagedObject, Codable {

    enum CodingKeys: String, CodingKey {
        case id
        case title
        case purchaseDate
    }

required convenience public init(from decoder: Decoder) throws {

var context : NSManagedObjectContext = MyAppCoreDataManager.sharedInstance.persistentContainer.viewContext
        guard let entity = NSEntityDescription.entity(forEntityName: "Purchase", in: context!) else { fatalError() }

        self.init(entity: entity, insertInto: context!)
        let container = try decoder.container(keyedBy: CodingKeys.self)

        self.title = try container.decodeIfPresent(String.self, forKey: .title)
        self.id = try container.decodeIfPresent(String.self, forKey: .id)
        self.purchaseDate = try! container.decodeIfPresent(Date.self, forKey: .purchaseDate)

    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(id, forKey: .id)
        try container.encode(title, forKey: .title)
try container.encode(purchaseDate, forKey: .purchaseDate)
}
}

//   Purchase+CoreDataProperties.swift

extension Purchase {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Purchase> {
        return NSFetchRequest<Purchase>(entityName: "Purchase")
    } 


    @NSManaged public var id: String?
    @NSManaged public var title: String?
    @NSManaged public var purchaseDate: Date?
}

//CALL

   let decoder = JSONDecoder()

                    decoder.dataDecodingStrategy = .base64
                    let purchases = try decoder.decode([Purchase].self, from: value)

If i remove "purchaseDate" everything works correctly