using bluebird with undefined success callback function

I am using bluebird library over memcached.

memcached.set('foo', 'bar', 10, function (err) { /* stuff */ });

this function does not call success callback in second parameter so seems like .then(res) function does not getting called.

 Promise.promisifyAll(memcached);
 memcached.setAsync(hashedCacheKey, obj).then(function (res) {
            resolve(res);
        }).catch(function (err) {
            reject(err, null);
        });

is there any way for me to handle uncalled success event?