Validation error for Extra registration fields

Hello, I’ve added some extra fields to the registration page (adding some code in “function.php” like many tutorials explain) and it works with the classic registration page.
With the Modal Plugin instead, it is not able to verify the fields and even if you’ve correctly filled them, it gets stuck on the first custom field (like it was empty). Any suggestion? Thanks.
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http://wordpress.org/plugins/wp-modal-login/