Receiving a fatal error message when logging in as admin

When I log in to my site, I receive the following fatal error message:

Catchable fatal error: Argument 2 passed to MS_Controller_Frontend::propagate_ssl_cookie() must be an instance of WP_User, none given in /home/thedajd2/public_html/wp-content/plugins/protected-content/app/controller/class-ms-controller-frontend.php on line 818

When I log in as a subscriber (ie. under a dummy account), this message does not appear.

I had not experienced this problem until I added the Pie Register plugin (which I needed so I could add custom registration fields; BuddyPress wasn't working with Protected Content).

I still log in, in spite of the error, and am able to move around the site and interact with it as per usual. Not sure what's causing the error message or how I can remove it from my WordPress experience.