Team Calendar subscribe url is broken

[Chris Benson]

When trying clicking the subscribe button for our calendar it gives the following:

https://domain/rest/calendar-services/1.0/calendar/export/subcalendar/undefined.ics?os_authType=basic&isSubscribe=true

but for some reason it has undefined in the url and obviously doesn't work, manually looking at the html it should be generating the url:

https://domain/rest/calendar-services/1.0/calendar/export/subcalendar/5185f79c-86ef-48ca-ab4d-a3afd7f8b06e.ics?os_authType=basic&isSubscribe=true

which does work fine but our users are getting frustrated at having to ask what the correct url is. 

Is there any obvious reason it's doing this?

[AnnWorley]

This is my initial workaround answer while I research further.

I noticed the URL is no longer "undefined" when I use the ellipses menu instead of the button.

Button result:

Ellipses result:

Hopefully this will serve as a workaround while we investigate further. Please let me know if it helps at all.