Fatal Error

Hi I get this error when installing the MU Support System plugin on my standard Wordpress installation, version 3.1.2. Please help.

Plugin could not be activated because it triggered a fatal error.

Fatal error: Call to undefined function get_current_site() in /hermes/web04/b1588/moo.deepervibes/deepervibes1/wp-content/plugins/incsub-support/incsub-support.php on line 255