Fatal Error when Adding New Order

Just curious if anyone has seen the following error:

Fatal error: Call to a member function is_download_only() on a non-object in /home/machin34/public_html/wp-content/plugins/marketpress/includes/admin/class-mp-orders-admin.php on line 307

This is happening when trying to add a new order from the dashboard. Any idea what this is referring to so I can begin to troubleshoot?