mu.php error

Ok, so I don't read code. Can someone tell me what this error msg means? I'm getting it across all the blogs. It shows at the bottom of the screen but I can't see that it's affecting any functionality. Wouldn't know it's there unless I look way down at the bottom of the screen below the WPMU credits.

Warning: constant() [function.constant]: Couldn't find constant yes in /home/mydomain/public_html/wp-admin/includes/mu.php on line 718