getModified() returns object where ids are expected

<?php
require_once '_loader.php';

$mgr = Doctrine_Manager::getInstance();
$mgr->openConnection('....');

class User extends Doctrine_Record
{
    public function setTableDefinition()
    {
        $this->setTableName('t_users');

        $this->hasColumn('name', 'string', 32);
    }
}

class Post extends Doctrine_Record
{
    public function setTableDefinition()
    {
        $this->setTableName('t_posts');

        $this->hasColumn('body', 'string', 256);
        $this->hasColumn('userId', 'integer');
    }

    public function setUp()
    {
        $this->hasOne('User', array(
            'local'   => 'userId',
            'foreign' => 'id'
        ));
    }
}

Doctrine::dropDatabases();
Doctrine::createDatabases();
Doctrine::createTablesFromArray(array('User', 'Post'));

$joe = new User();
$joe->name = 'Joe';
$joe->save();

$post = new Post();
$post->body = 'Hi';
$post->User = $joe;
$post->save();

$bill = new User();
$bill->name = 'Bill';
//$bill->save(); // If this is uncommented everything works fine

$post->User = $bill;

var_dump(' --- New --- ');
var_dump($post->getModified());

string(13) " --- New --- "
array(1) {
  ["userId"]=>
  object(User)#37 (18) {
    ["_node:protected"]=>
    NULL
    ["_id:protected"]=>
    array(0) {
    }
    ["_data:protected"]=>
    array(2) {
      ["id"]=>
      object(Doctrine_Null)#4 (0) {
      }
      ["name"]=>
      string(4) "Bill"
    }
    ["_values:protected"]=>
    array(0) {
    }
    ["_state:protected"]=>
    int(2)
    ["_lastModified:protected"]=>
    array(0) {
    }
    ["_modified:protected"]=>
    array(1) {
      [0]=>
      string(4) "name"
    }
    ["_oldValues:protected"]=>
    array(1) {
      ["name"]=>
      NULL
    }
    ... trimmed ...

string(13) " --- New --- "
array(1) {
  ["userId"]=>
  string(1) "2"
}