[Plugin: WordPress Related Posts] Fatal Error on Activation

Once I activated the plugin it throws me this error and i cannot see my admin backend anymore.

Fatal error: Call to undefined function is_site_admin() in /home/xxxx/public_html/xxxxx/wp-content/plugins/wordpress-related-posts/related-posts.php on line 81

http://wordpress.org/extend/plugins/wordpress-23-related-posts-plugin/