ezcInputFormDefinitionElement problem

Hi there!

I'm looking for a hint on the following problem using the components, 
any help appreciated.
One of my templates provides a form with an input field called "password":

<form action="/authenticate" method="post">
	<fieldset>
		<input name="password" id="password" class="textinput" type="password" />
		<button type="submit">Login</button>
	</fieldset>
</form>

Any submit will call the action slot "authenticate" that will validate 
the form data according to my form definition:

private static function getFormDefinition() {

	$definition = array(
		'password' => new ezcInputFormDefinitionElement(
			ezcInputFormDefinitionElement::REQUIRED,
			'callback',
			array('appFormTools', 'checkEmpty')
		)
	);
	return $definition;

}

Here's the callback function for the password: