MongoWriteConcernException.不可变字段'_id'发现已被更改为_id

在CodeIgniter中更新MongoDB时，我有以下错误:

Type: MongoWriteConcernException
Message: localhost:27017: After applying the update to the document {_id: ObjectId('55ee98543bd7af780b000029') , ...}, the (immutable) field '_id' was found to have been altered to _id: "55ee98543bd7af780b000029"
Filename: C:'xampp'htdocs'CI'application'models'mongo_model.php

这是我的控制器代码

public function  update()
{
    $userdata['firstname'] = $this->input->post('txtfirstname');
    $userdata['lastname'] = $this->input->post('txtlastname');
    $userdata['email'] =  $this->input->post('txtemail');
    $userdata['password'] = md5($this->input->post('txtpassword'));
    $userdata['_id'] = $this->input->post('hiddenId');
    $collection=  $this->mongo_model->updateuserdb($userdata);
    if ($collection)
    {
        header('location:'.base_url()."index.php/user".$this->index());
    }
}

，模型代码为

public function updateuserdb($userdata)
{
    $id = $userdata['_id'];
    $collection = $this->mongo_db->db->selectCollection('myfirstCollection');
    $query = $collection->update(array('_id' => new MongoId($id)), array('$set' => $userdata), array('upsert' => FALSE));
    return $query;
}