PHP message: PHP Fatal error: Call to undefined function

PHP message: PHP Fatal error: Call to undefined function is_admin() in /home/nowme/public_html/wp-content/plugins/wpmu-dev-post-votes/lib/plugins/wdpv-checks-allow_daily_voting.php on line 35" while reading response header from upstream, client: 66.249.75.82, server: 20now.me, request: "GET /wp-content/plugins/wpmu-dev-post-votes/lib/plugins/wdpv-checks-allow_daily_voting.php HTTP/1.1