WHMCS WP Integration - post update errors

Greetings,

I've begun receiving the below error message since upgrading WHMCS WordPress Integration, and was wondering what you would advise that I do to resolve this:

Fatal error: Call to undefined function mcrypt_encrypt() in /home/stratusc/public_html/wp-content/plugins/whmcs-wordpresss-integration/whmcs-integration.php on line 354

Thank you,

Cole