[Plugin: Custom Contact Forms] Fatal error with WordPress 3.0 multiblog

Hi,

I use WordPress 3.0 multiblog. When activating the plugin within one of these blogs I get this error:

Fatal error: Call to undefined function wp_get_current_user() in /homepages/25/d13574424/htdocs/multisites/wp-includes/capabilities.php on line 1059

Any idea what to do?
Thanks

http://ww.wp.xz.cn/extend/plugins/custom-contact-forms/