What is causing this error?

I have the following code:            $c1 = new Criteria();            $c1->add(UsersPeer::ID,$Username);            $rs = UsersPeer::doSelect($c1);Which gives this error when I call it:Fatal error: Call to undefined method Users::hydrate() in /var/www/vhosts/httpdocs/src/db/build/classes/Link/om/BaseUsersPeer.php on line 448I've looked and looked but cannot find the cause of this error.  Any help would be greatly appreciated.G-Man