I can't seem to add an overlay for a calendar from my main site. I have a site (companyname.com) and several department subsites, and both the main site and each department subsite has a calendar. When I enter the URL to resolve a subsite for any calendar overlay (eg - https://companyname.com/billing) it works and I can select a list and a view. This works whether I am creating an overlay from the main site calendar or any of the subsite calendars. But when I try to resolve the address for the main site nothing seems to work. I have tried the site url, /lists, the full url of the calendar, and nothing seems to work.
For reference, the calendar is an app on the main site. It is called Company Calendar. There are no special permissions or anything, and all other calendars are built in the same way but on subsites.