Set method in Yii 1.1 data caching system always returns false value

Yii 1.1 General Discussion
1

We have a web application which is based on Yii 1.1 . I want to cache some data so I found Yii documentation about data caching and I did it step by step but set method always return false when I debug it.
First I configured cache components (and then enabled memcache extension on my host):

'cache'=>array(
        'class'=>'system.caching.CMemCache',
        'servers'=>array(
            array('host'=>'server1', 'port'=>11211, 'weight'=>60),
            array('host'=>'server2', 'port'=>11211, 'weight'=>40),
        ),
    ),

And here is my sample code for caching but get method always returns false so when I debugged it with an if condition I found set method doesn’t set cache data and returns false value .Note that $merchant_id is a unique number.

public static function getMerchantLogo($merchant_id='',$logo='')
{    	
	//my codes to get $merchant_logo by their merchant_id

	$cache=Yii::app()->cache->set($merchant_id, $merchant_logo);
	if($cache){
	   return $merchant_logo;
	}else{
	    //set $merchant_logo a  default value
	  return $merchant_logo;  
	}
}