Client using this just lost #1 on Google SERP

The following bug causes 500 errors to show to Google/Yahoo/Bing crawlers meaning the whole site will be unindexed.

PHP Fatal error: Call to undefined function is_user_logged_in() in /public_html/wp-content/plugins/xslt/xslt.php on line 50

A fix is to add this line to the top of the xslt.php file.

include_once(ABSPATH . ‘wp-includes/pluggable.php’);