CakePHP pagination with HABTM models

I'm having some problems with creating pagination with a HABTM relationship. First, the tables and relationships:

requests (id, to_location_id, from_location_id)
locations (id, name)
items_locations (id, item_id, location_id)
items (id, name)

So, a Request has a Location the request is coming from and a Location the Request is going to. For this question, I'm only concerned about the "to" location.

Request --belongsTo--> Location* --hasAndBelongsToMany--> Item

(* as "ToLocation")

In my RequestController, I want to paginate all the Items in a Request's ToLocation.

// RequestsController
var $paginate = array(
    'Item' => array(
        'limit' => 5,
        'contain' => array(

// RequestController::add()
$locationId = 21;
$items = $this->paginate('Item', array(
    "" => $locationId

And this is failing, because it is generating this SQL:

SELECT COUNT(*) AS count FROM items Item   WHERE = 21

I can't figure out how to make it actually use the "contain" argument of $paginate...

Any ideas?