get_taxonomies by post_type no output

[tangfou]

if:

<?php
register_taxonomy('area', array('demand'), $args);

$taxonomies = get_taxonomies( 'object_type' =>array('demand'), $output, $operator );

that is ok!

but if:

<?php

register_taxonomy('area', array('profile', 'resource', 'demand'), $args);

$taxonomies = get_taxonomies( 'object_type' =>array('demand'), $output, $operator );

There is no output.