BRISK Feature Detector Detects Zero Key Points

The Brisk metric shown below does not give me any key points. Can someone suggest a problem. I will try to explain what I am doing below with some codes.

#include "opencv2/features2d/features2d.hpp"

using namespace cv;    
using namespace std;

Ptr<FeatureDetector> detector;
detector =  FeatureDetector::create("BRISK");

// the filename is given some path

Mat img = imread(filename, 0);
CV_Assert( !img.empty() );

vector<KeyPoint> kp;

detector->detect(img, kp);

So, when I debug this and check the key point (kp), it says <0 items>

with the same code, another detector, such as ORB, SIFT or SURF, works as intended!

Can someone suggest a solution.

I am using opencv 2.4.9 with the creator of Qt 2.5.2

thank