Fatal error: wp_get_mu_plugins

When trying to activate the Wordpress Membership plugin on a Wordpress 3 website (not MU), I get the following error:

Plugin could not be activated because it triggered a fatal error.
Fatal error: Call to undefined function wp_get_mu_plugins() in /var/home/websites/nasmm.org/www/wp-content/plugins/membership/membership.php on line 27