Fatal Error When Setting Up Multisite

Hello,

I am attempting to set my site up as a multisite, but when I add:

/* Multisite */
define( ‘WP_ALLOW_MULTISITE’, true );

to my wp-config.php file and then try to navigate back to my site I get a ton of letters and numbers and this message in bold:

Call to undefined function nocache_headers() in /home/norpac/public_html/wp-admin/admin.php on line 33

How do I fix this?