Can't Get `fetchAllSubscriptionsOperation()` to Work

I'm trying to get all the CKSubscriptions associated with the current user. I am trying this:

        let op = CKFetchSubscriptionsOperation.fetchAllSubscriptionsOperation()
        op.fetchSubscriptionCompletionBlock = { (subs, error) in
            print("*** fetched subs: \(subs)")
        }
        let q = OperationQueue()
        q.addOperation(op)

This call doesn't seem to work properly: I get an empty dictionary in the `subs` parameter. The `error` param is nil.

I happen to know that subscriptions for the user exist because calling `publicDatabase.fetchAllSubscriptions()` function works as I expect: it returns all subscriptions in the schema, including the current user's.

Can anyone let me know if I am doing something wrong or if there is some other way to get this information from CloudKit?

Thank you