Current limitations and workarounds.
When you subscribe to a newly created agent's calendar URL in Google Calendar, it may show an error like “Could not fetch the URL” or silently fail. This happens because Google Calendar rejects iCal feeds that contain zero events.
Create at least one event on the agent's calendar before subscribing in Google Calendar. Every new agent already has a birthday event, so this should work out of the box. If you deleted the birthday event, create any event first, then add the subscription URL.
Apple Calendar and Outlook handle empty feeds without issues. If you're primarily using one of these apps, no workaround is needed.