Activity.get returns source_contact_id, but ignores it in requests

With an Activity.get call, the values returned include the field source_contact_id. When source_contact_id is passed to Activity.get as a parameter, it is ignored. Records that are returned of course include those with that source_contact_id, but also others.

So a request with parameters like:

$params = array(
'version' => 3,
'sequential' => 1,
'source_contact_id' => 67,
);

returns results including:

{ "id":"2", "activity_type_id":"10", "subject":"Subject for Pledge Acknowledgment", "activity_date_time":"2013-12-28 12:15:18", "duration":"1", "status_id":"2", "is_test":"0", "is_auto":"0", "is_current_revision":"1", "is_deleted":"0", "source_contact_id":"66" }

,

Replicated on 4.4 and 4.5 demo API explorer.