my authentication fails and when i try to getDbSelect() i get nothing … what did i do wrong?

class AuthController extends Zend_Controller_Action
{
    protected $auth;

    public function init()
    {
        $db = $this->getInvokeArg("bootstrap")->getResource("db");
        $this->auth = new Zend_Auth_Adapter_DbTable($db, 'users', 'username', 'password', 'md5(?)');
    }
    public function loginAction()
    {
        if (isset($_POST["username"])) {
            $result = $this->auth
                ->setIdentity($this->getRequest()->getParam('username'))
                ->setCredential($this->getRequest()->getParam('password'));
            echo " > " . $this->auth->getDbSelect();die();
            $result = $this->auth
                ->setIdentity($this->getRequest()->getParam('username'))
                ->setCredential($this->getRequest()->getParam('password'))
                ->authenticate();
            if ($result->isValid()) {
                $this->_redirect("postLogin");
            }
        }
    }

Update 1:

i also tried without MVC and with MySQL which i am more familiar with:

// setup auto loading
require_once 'Zend/Loader/Autoloader.php';
Zend_Loader_Autoloader::getInstance();

// setup db connection
$db = new Zend_Db_Adapter_Pdo_Mysql(array(
    'host'     => '127.0.0.1',
    'username' => 'root',
    'password' => '',
    'dbname'   => 'learnZendAuth'
));

// create authentication adapter
$auth = new Zend_Auth_Adapter_DbTable($db, 'users', 'username', 'password', 'md5(?)');

// do login if request is posted
if (isset($_POST) && !empty($_POST)) {
    // do authencation
    $result = $auth->setIdentity($_POST['username'])
         ->setCredential($_POST['password'])
         ->authenticate();

    // check authentication valid
    if ($result->isValid()) {
        // login success
        echo "successfully logged in as: " . $result->getIdentity();
    } else {
        // login fail
        echo "
";
        print_r($result->getMessages());
        echo "
";

    }

} else {

    // request not posted

    // see if already logged in

    if (Zend_Auth::getInstance()->hasIdentity()) {

        echo "already logged in as: " . Zend_Auth::getInstance()->getIdentity();

    }

}
i get 

Fatal error: Uncaught exception

  'Zend_Auth_Adapter_Exception' with

  message 'The supplied parameters to

  Zend_Auth_Adapter_DbTable failed to

  produce a valid sql statement, please

  check table and column names for

  validity.' in C:\Program Files

  (x86)\Zend\ZendServer\share\ZendFramework\library\Zend\Auth\Adapter\DbTable.php:414

  Stack trace: #0 C:\Program Files

  (x86)\Zend\ZendServer\share\ZendFramework\library\Zend\Auth\Adapter\DbTable.php(306):

  Zend_Auth_Adapter_DbTable->_authenticateQuerySelect(Object(Zend_Db_Select))
1 C:\Program Files (x86)\Zend\Apache2\htdocs\learnZf\index.php(23):
Zend_Auth_Adapter_DbTable->authenticate()
2 {main} thrown in C:\Program Files (x86)\Zend\ZendServer\share\ZendFramework\library\Zend\Auth\Adapter\DbTable.php
on line 414