BP Activity plus is producing errors in my error log

Any ideas what might be causing this? I'm running WP Multisite with BP, both fresh installs running the latest version on Bluehost.

/home1/USER/public_html/wp-content/plugins/buddypress-activity-plus/lib/error_log:

[22-Aug-2011 22:08:07] PHP Fatal error: Call to undefined function add_action() in /home1/USER/public_html/wp-content/plugins/buddypress-activity-plus/lib/bpfb_group_documents.php on line 217

http://imgur.com/7m66l

Thanks!