Membership2 - cannot assign subscription to admin?

Hi, I'm setting up a website where some of the members will also run the site. I can't seem to assign a sub to an admin - attempts to search for admin users in 'add members' comes up blank, while adding a sub to a member via the members list (and modding start/end dates) comes up with the following error:

Warning: Creating default object from empty value in ###/content/plugins/membership/app/controller/class-ms-controller-member.php on line 401

Fatal error: Call to undefined method stdClass::save() in ###/content/plugins/membership/app/controller/class-ms-controller-member.php on line 404

(this is a demo setup, with debug turned on)
Is this a bug / setup error? Or is this intentionally blocked?