Admin Subscription within Membership 2

Not sure if I am going about this the right way but I cannot see why it shouldn’t work but it doesn’t.

I want to restrict access to certain pages and menus to admin users only and can’t seem to see any way of adding them without creating a subscription for admins. So I created a free subscription that has permanent access. Yet when I try to add a user manually to this subscription I get through to the stage of Edit Member where it has the fields such as start date etc.. Yet it shows a expiry date even though I have set permanent access. Selecting a date for this and then clicking save then returns a 500 Internal Server page. Can you advise if I am doing this wrong, or if not why can I not add a user to this subscription?