Pro Sites is keeping deactivating itself

This is not the first time I've discovered that Pro Sites had magically disabled itself! The debug.log shows a database error that might be related:

WordPress database error Incorrect index name for query ALTER TABLE wp_pro_sites ADD KEY ' (blog_ID,level,expire) made by require('wp-admin/plugins.php'), activate_plugin, include_once('/plugins/pro-sites/pro-sites.php'), ProSites->__construct, ProSites->install, dbDelta, QM_DB->query