Codeigniter 3 memcached issue

i’m getting this error in my drivers/Cache_memcached.php file

A PHP Error was encountered
Severity: Warning

Message: Invalid argument supplied for foreach()

Filename: drivers/Cache_memcached.php

Line Number: 120

i have downloaded 5.6 memcached ptted in php/ext/ but i’m getting this error in CI3 with php 5.6
i’m getting $this->_config = NULL value

please help me out anyone thanx in advance