Fatal error after activation of the plugin

Fatal error: Uncaught Error: Call to undefined function wp_get_current_user() in /var/www/wp-includes/capabilities.php:640 Stack trace: #0 /var/www/wp-content/plugins/wp-toolbelt/index.php(17): current_user_can(‘manage_options’) #1 /var/www/wp-settings.php(362): include_once(‘/var/www/wp-con…’) #2 /var/www/wp-config.php(85): require_once(‘/var/www/wp-set…’) #3 /var/www/wp-load.php(37): require_once(‘/var/www/wp-con…’) #4 /var/www/wp-admin/admin.php(34): require_once(‘/var/www/wp-loa…’) #5 /var/www/wp-admin/update.php(14): require_once(‘/var/www/wp-adm…’) #6 {main} thrown in /var/www/wp-includes/capabilities.php on line 640