Sign Up Language error

I was testing the sign up page and i get the following errors:

Warning: include_once(/home/shortsal/public_html/wp-admin/includes/mu.php) [function.include-once]: failed to open stream: No such file or directory in /home/shortsal/public_html/wp-content/mu-plugins/signup-language.php on line 151

Warning: include_once() [function.include]: Failed opening '/home/shortsal/public_html/wp-admin/includes/mu.php' for inclusion (include_path='.:disappointed:usr/local/php5/lib/php') in /home/shortsal/public_html/wp-content/mu-plugins/signup-language.php on line 151

What do I need to fix?