PHP Fatal error: Call to undefined function mb_strlen() in /var/www/heritage/components/com_comment/joscomment/utils.php on line 575, referer: http://hostname/administrator/index.php

PHP Fatal error: Call to undefined function mb_strlen() in /var/www/heritage/components/com_comment/joscomment/utils.php on line 575, referer: http://hostname/index.php?option=com_content&view=article&id=54:2009-03-25-02-51-43&catid=1:latest-news

I've download and install 4.0 yesterday.
This error appears in logs after 2-nd comment (maybe after first -- I'm not sure)



System:
Linux ***.ua 2.6.22.9-91.0.120asp #1 SMP Tue Oct 2 00:17:42 EEST 2007 i686
MySQL: 5.0.45
utf8_general_ci
PHP: 5.2.4
Apache/2.2.6 (ASPLinux)
-> PHP: apache2handler
Joomla: Joomla! 1.5.9 Production/Stable [ Vatani ] 9-January-2009 23:00 GMT