Fatal error: Call to undefined function switch_to_blog()

I've got the error below when trying to install 'Pro Sites' on a fresh/clean install of WP 3.9.1 (PHP 5.5). No other themes/plugins installed.

Any idea what causes this?

Fatal error: Call to undefined function switch_to_blog() in /Users/andy/Sites/WP/wptest/wp-content/plugins/pro-sites/pro-sites.php on line 185