[Affiliates] Fatal error activating account from email

I think this is caused by MarketPress or Affiliates, but when I invite a user to the system, and they click through to wp-activate.php, they get a 500 error. The logs show:

FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught Error: Class 'WPMUDEV_Metabox' not found in /var/www/wp-content/plugins/affiliate/affiliateincludes/addons/marketpress.php:126

Any ideas?