problem with the AVH plugin

Can't comment on a certain blog, found this in my error log:

[Tue Jun 14 20:54:52 2011] [warn] [client] mod_fcgid: stderr: PHP Fatal error:  Call to undefined function is_set() in /var/www/clients/clientxx/webxx/web/wp-content/mu-plugins/avh-files/class/avh-fdas.public.php on line 370, referer:

The blog was a splog, gonna delete it soon but maybe you can help me fix the problem with the plugin?