translation doesn't work with second field name in "matches" validation rule

[eluser]Benedikt[/eluser]
<code>
$rules[] = array( 'field' => 'Password', 'label' => 'langassword', 'rules' => 'trim|required|matches[Password2]' );
$rules[] = array( 'field' => 'Password2', 'label' => 'langassword2', 'rules' => 'trim|required' );
</code>

<code>
$lang['Password'] = "Password";
$lang['Password2'] = "Confirm password";
</code>

What is wrong then? In all error messages (no matching-rule here) it works, but here no.

What am I doing wrong then?

<code>
// CI Version
define('CI_VERSION', '1.7.2');
</code>