Inherit from EKCalendarItem

Question

Because EKEvent can not hold extra properties, I was thinking of creating my own class Event and inherit from EKCalendarItem (same as EKEvent class).

But I get a FrozenClass error, which is quite new to me. Does anybody have any idea of what that means? EKCalendarItem is an open class, so as far as I know I should be able to inherit from that. Or... am I wrong here?

The exact error:

'+[MyApp.Event frozenClass]: unrecognized selector sent to class 0x105667068'

My code:

class Event: EKCalendarItem {

// MARK: - Properties

var id: String
var startDate: Date
var endDate: Date
var isAllDay: Bool

// MARK: - Inits

init?(id: String, dictionary: [String: Any]) {
    guard
        let title = dictionary["title"] as? String,
        let startDate = dictionary["startDate"] as? Timestamp,
        let endDate = dictionary["endDate"] as? Timestamp,
        let isAllDay = dictionary["isAllDay"] as? Bool
        else { return nil }

    self.id = id
    self.startDate = startDate.dateValue()
    self.endDate = endDate.dateValue()
    self.isAllDay = isAllDay

    super.init()

    self.location = dictionary["location"] as? String
    self.title = title
    self.notes = dictionary["notes"] as? String
}

convenience init?(snapshot: QueryDocumentSnapshot) {
    self.init(id: snapshot.documentID, dictionary: snapshot.data())
}

}