Fatal Error adding membership class-ms-addon-searchindex.php on line 286

I've had a couple of occurrences of this error today. Any idea what might be causing it? I had a problem with a new member yesterday apparently not being able to access protected content, despite having the appropriate membership assigned in 'all members'. Could these be related?

mod_fcgid: stderr: PHP Fatal error: Call to a member function add_membership() on a non-object in ...../wp-content/plugins/membership/app/addon/searchindex/class-ms-addon-searchindex.php on line 286