Random hydratation problem

I have the following collection named Isboo_Item :

{
"_id": ObjectId("4d3d02fec9fa877d44a36978"),
"author": null,
"backpack": null,
"itemType": 1,
"owner": "4d3cf450b532519765080000",
"place": null,
"quality": null,
"quantity": 1,
"value": null
}

I have a such PHP class :

/**

• @Document(db="Isboona", collection="Isboo_Item")
• */
  class Isboo_Item extends Isboo_Db
  {
  (...)

When I'm trying :
$dm->findOne($collection, array('_id' => $id));

I get :

Fatal error: Uncaught exception 'ReflectionException' with message 'Class 4 does not exist' in /users/developpement/www/library/Doctrine/ODM/MongoDB/Mapping/ClassMetadata.php:285

However the same works fine with another collection :

{
"_id": ObjectId("4d3cf450b532519765080000"),
"characs":

{ "mana": 64, "strenght": 83, "agility": 69, "dexterity": 88, "constitution": 94, "culture": 79, "vivacity": 57, "eloquence": 64, "intellect": 74, "willpower": 88, "empathy": 96, "charisma": 83 }

,
"name": "Krok",
"position":

{ "IDplace": 0, "posX": 0, "posY": 0, "posLocalX": 0, "posLocalY": 0 }

,
"refFather":

{ "$ref": "Isboo_Character", "$id": ObjectId("4d3cf450b532519765090000"), "$db": "Isboona" }

,
"refMother":

{ "$ref": "Isboo_Character", "$id": ObjectId("4d3cf450b5325197650a0000"), "$db": "Isboona" }

}