Thread:Extension talk:MobileFrontend/Html Formatter Not Found

With the simple configuration (Mediawiki v1.21) I get a 500 error for all pages. The error is:

PHP Fatal error: Class 'HtmlFormatter' not found in /var/www/internal/wiki/extensions/MobileFrontend/includes/formatters/MobileFormatter.php on line 6...

Is this something that can be fixed with configurations, or a compatibility issue