"IN" operator doesn't work on the contact_sub_type field for api.Contact.get

civicrm_api3('Contact', 'get', array(
  'sequential' => 1,
  'contact_sub_type' => array('IN' => array("Parent")),
));

... always returns zero contacts. As a result, it is impossible for search for contacts who have a contact subtype of either Parents or Teachers.