Membership: trouble upgrading from default free subscription to paid

I use a different registration redirect module that forces administrator intervention for all registrations, so I have not used the registration mechanism from Membership. Registrants are put into a free subscription called "Members", and they can choose to change to a paid membership at any time. At the moment the "Members" subscription is set to private; all others are public. I have the "free subscriptions" and "paypal express" gateways activated.

When I attempt to subscribe to one of the subscriptions using the form displayed on the "subscriptions" page (as listed under Membership Page Options), I see the correct subscription information shown on the PayPal login screen. When I log in to PayPal, though, I get this message:

"We are unable to modify your subscription. This may be because your existing subscription calls for no more payments to be made or because it has been cancelled or completed."

I am not sure what I need to do to configure Membership to avoid this.