API: chaining using a custom contact reference field

In our implementation, we have a custom field 'organizer' (custom_56) on events, which is a contact reference. I tried to use the API to get a contact, and (using chaining) all events that have this contact as organizer.

I tried the following:

$result = civicrm_api3('Contact', 'get', array(
'sequential' => 1,
'external_identifier' => "leg/0403",
'api.Event.get' => array('custom_56_id' => "\$value.id"),
));

It did not work, I got all events in api.event.get. I tried with 'custom_56' (without _id) as well, it did not work either.

So I guess this is a bug.