MongoCollection::update() Warning

Am Using Codeigniter with mongodb. I got this Following Erro when i update my code. 

A PHP Error was encountered
Severity: Warning
Message: MongoCollection::update(): The value of the 'w' option either needs to be a integer or string
Filename: libraries/Mongo_db.php
Line Number: 1241
Backtrace:
File: /localhost/mongo/application/libraries/Mongo_db.php
Line: 1241
Function: update
File: /localhost/mongo/application/models/Login_model.php
Line: 80
Function: update
File: /localhost/mongo/application/controllers/Login_Controller.php
Line: 91
Function: updateUserDetails
File: /localhost/mongo/index.php
Line: 315
Function: require_once


What can i do for this type of warning. And This Error Only Occur in live server.

My update Coding is - 
$userId    = '58ff651dd98a860c15000029';
$data = array('PROFILE_PHOTO'=>$picName);
$this->mongo_db->where('_id', new MongoId($userId))->set($data)->update('user_details');