Can't unsubscribe from Time feed

> The Circuitpython API for Adafruitio's MQTT client has a method called `subscribe_to_time("iso")`. But it does not appear to have a method to unsubscribe. It appears to subscribe to `time/ISO-8601`. If I try to use the `unsubscribe()` method on that feed name (or just `ISO-8601`) an exception is raised saying you must subscribe first. How can I unsubscribe from the time feed?