Trying to Update Calendar, Getting Various Errors

I can add a new calendar to the user's calendars like this, using the saveCalendar(_:commit:) method:

let ekStore = EKEventStore()

func saveCalendar(calendar: EKCalendar) throws {
    try ekStore.saveCalendar(calendar, commit: true)
}

let newList = EKCalendar(for: .reminder, eventStore: store.ekStore)

newList.source = store.ekStore.defaultCalendarForNewReminders()?.source
newList.title = newListName
newList.cgColor = listColor

do {

    try saveCalendar(calendar: newList)

} catch {

    print("Error adding list: \(error.localizedDescription)")

}

And I store the calendar object.

When the user finishes editing a calendar (reminders list) in my app, I try to save it like this, using the stored calendar as a starting point:

let updatedList = existingCalendar

updatedList.title = newListName
updatedList.cgColor = listColor

do {

    try saveCalendar(calendar: updatedList)

} catch {

    print("Error saving list: \(error.localizedDescription)")

}

But the calendar doesn't save and I get this error: That account does not support reminders..

I have also tried explicitly setting the calendar's source:

updatedList.source = store.ekStore.defaultCalendarForNewReminders()?.source

but then I get this error: That calendar may not be moved to another account..

My Question: How do I update calendars (reminder lists) from my app?