无法使zend_filter_输入正常工作

我正在尝试让zend_filter_输入在一个简单的登录表单上按需要工作。

这是我的密码

    $filters = array('username' => 'StringTrim', 'password'  => 'StringTrim');  
    $validators  = array( 
              'username' => array('Alnum', 'presence' => 'required'),
              'password' => array('Alnum', 'presence' => 'required')
    );

    $input = new Zend_Filter_Input($filters,$validators);

    print_r($input->getMissing());

答案是

Array
 (
    [username] => Array
    (
        [0] => Field 'username' is required by rule 'username', but the field is missing
    )

    [password] => Array
    (
        [0] => Field 'password' is required by rule 'password', but the field is missing
    )

  )

我收到了官方文件。为什么这么说 rule "username" and rule "password" 在这里?

谢谢