Error when commenting on article

Hi there,

When users try to fill in a comment on an article on our
website(http://www.nieuwsbedrijf.
nl), they get an error like this:

Fatal error: Call to a member function on a non-object
in /usr/local/campsite/www-common/html/comment_lib.php on
line 153

Does anyone of you have an idea on how to solve this?

Regards,

Jeroen van de Nieuwenhof