registration plugin help

I am working on a plugin for the registration process. This is a function that should add it’s own error to the existing errors when registering a new account. I don’t get an error but it doesn’t get added to the list of errors on the login page either. Can anyone help me with this?

function compareEISnumber(){
$errors = new WP_Error();
$errors->add('wrong_license', __('<strong>ERROR</strong>: This is not a valid license number.'));

$errors = apply_filters( 'registration_errors', $errors );
if ( $errors->get_error_code() )
		return $errors;

}
add_action('register_post', 'compareEISnumber');

It called but does not add the