GroupContact doesn't understand group_id variable

I'm using GroupContact to work out whether someone is a member of a group, e.g.

http://sandbox.drupal.civicrm.org/civicrm/ajax/rest?json=1&debug=1&version=3&entity=GroupContact&action=get&group_id=2&contact_id=28 (I defined group_id=2&contact_id=28)

I am expecting either 0 or 1 results i.e. yes they are in the group or no they aren't.

But instead I get 2. I also don't get the contact_id in the result.

In fact it looks like I am calling the wrong API here. It looks like it is calling Group, not GroupContact.

When I do

http://sandbox.drupal.civicrm.org/civicrm/ajax/rest?json=1&debug=1&version=3&entity=GroupContact&action=get&Group_id=2

I get something that looks much more like what I would want.