API throws "undefined_fields" ["contact_id.gender_id"]

I was very happy when I learned that CiviCRM 4.7 would support api joins.
(https://issues.civicrm.org/jira/browse/CRM-17795 which has been implemented)
For our purpose I wanted to retrieve the number of male & female participants of an event.

Unfortunately there seems to be a glitch with the gender field:

When I try (through the api explorer locally and on the 4.7 sandbox):
$result = civicrm_api3('Participant', 'get', array(
'debug' => 1,
'event_id' => 3,
'contact_id.gender_id' => "Male",
'options' => array('limit' => 9999),
));

I get the debug message:
"undefined_fields":["contact_id.gender_id"]
and it disregards the gender filter