If I add the prefix to the element, it works, but to the form, it doesn't.

For example,

$testing = new Zend_Form_Element_Text('testing');
$testing->setLabel('Test Val')
->setRequired(true);

//new code
$testing->addPrefixPath('My','My/');

$testing->addValidator('TestVal');

$testing->setValidators($validators);

Works. Can anyone tell me why it doesn't work when I add it to form itself?