Attempting to save CKQuerySubscription yields 503 error

I'm building a CloudKit app, and I'm trying to get a subscription working for the iOS app. Subscriptions work in the web app. This code gives me a "Request failed with http status code 503".

myContainerId is the container ID in CloudKit Dashboard.

Code Block 
				let predicate = NSPredicate(value: true)
				let subscription = CKQuerySubscription(recordType: "Task", predicate: predicate, subscriptionID: "ios-tasks", options: [ CKQuerySubscription.Options.firesOnRecordCreation, CKQuerySubscription.Options.firesOnRecordUpdate, CKQuerySubscription.Options.firesOnRecordDeletion])
				subscription.zoneID = CKRecordZone.ID(zoneName: "com.apple.coredata.cloudkit.zone", ownerName: CKCurrentUserDefaultName)
				let info = CKSubscription.NotificationInfo()
				info.shouldSendContentAvailable = true
				subscription.notificationInfo = info
				
				let aContainer = CKContainer(identifier: myContainerId)
				
				let operation = CKModifySubscriptionsOperation(subscriptionsToSave: [subscription], subscriptionIDsToDelete: [])
				operation.modifySubscriptionsCompletionBlock = { (sub, subIds, error) in
						guard error == nil else {
								if let error = error {
										print("error while saving subscription: \(error.localizedDescription)")
								}
								return
						}
						
						print("Saved subscription!")
				}
				
				operation.qualityOfService = .utility
				aContainer.privateCloudDatabase.add(operation)

I'm primarily using an NSFetchedResultsController, and that's working great. When I go to CloudKit Dashboard, I see entities stored in my private database under the com.apple.coredata.cloudkit.zone zone.

I'm guessing this is a configuration issue or something?