Fatal Error on line 1403 when confirming singup

Hello,

I'm running some signup tests and have a constant fatal error message each time I click on confirm email :
Fatal error: Call to undefined method wpdb::tables() in /home/unecerta/public_html/wp-admin/includes/upgrade.php on line 1403

In case this would be relevant, I'on on wp 3.0.1.

Thanks for your help.
Olivier