Persisting a document which references another document drops ID

I have two documents:

@Document
class A {
/** @Id */
public $id;
/** @ReferenceOne */
public $b;
}

@Document
class B {
/** @Id */
public $id;
}

I have an already persisted instance of B, which A references,
though while persisting A and examining the resulting collection the id of B is missing.

{
_id: '000123',
b: {
$db: 'mydb',
$id: '',
$ref: 'b'
}
}

I debugged the code and found the error occurs in DocumentPersister.php at line 210 in the latest git version as of this posting.
The code is:

$inserts[$oid] = $data;

where $data is:

array(
_id => MongoId(000123),
b => array(
$ref => 'b',
$id => MongoID(123000),
$db => 'mydb'
)
)

and $inserts[$oid] becomes:

array(
$oid = array(
_id => MongoId(000123),
b => array(
$ref => 'b',
$id => MongoID(),
$db => 'mydb'
)
)
)

This assignment makes the MongoID disappear somehow!!?

Can't figure it out, could this be a problem with PHP itself?