imagettftext() error – captcha image doesn’t show up

I’ve installed the plugin and edited my comments.php file to add the captcha call. When I load the page I get the image not found [x] and the following error is generated in the /cryptographp/crypt/ folder—

[24-Jun-2007 11:13:34] PHP Fatal error:  Call to undefined function  imagettftext() in /home/hunter/public_html/blog/wp-content/plugins/cryptographp/crypt/cryptographp.inc.php on line 80
[24-Jun-2007 11:15:31] PHP Fatal error:  Call to undefined function  imagettftext() in /home/hunter/public_html/blog/wp-content/plugins/cryptographp/crypt/cryptographp.inc.php on line 80

Is this something I’m doing or…?