SURF Feature Detection in OpenCV 2.4.9

Question

I am using OpenCV 2.4.9 with Microsoft Visual Studio Community 2013 in Windows 8.1 to implement feature extraction using SURF. On debugging, I get this message : "Debug Assertation Failed. Expression : Invalid Null Pointer" Besides the command line does not take any arguments since the readme() function executes. I'm new to opencv and have thus used the code from the opencv 2.4.9 tutorial. Please help regarding this

#include <stdio.h>
#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/nonfree/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/nonfree/nonfree.hpp"
using namespace cv;
void readme();
/** @function main */
int main(int argc, char** argv)
{
    if (argc != 3)
    {
        readme(); 
    }
    Mat img_1 = imread(argv[1], CV_LOAD_IMAGE_GRAYSCALE);
    Mat img_2 = imread(argv[2], CV_LOAD_IMAGE_GRAYSCALE);
    if (!img_1.data || !img_2.data)
    {
        std::cout << " --(!) Error reading images " << std::endl; return -1;
    }

    //-- Step 1: Detect the keypoints using SURF Detector
    int minHessian = 400;
    SurfFeatureDetector detector(minHessian);
    std::vector<KeyPoint> keypoints_1, keypoints_2;
    detector.detect(img_1, keypoints_1);
    detector.detect(img_2, keypoints_2);
    //-- Draw keypoints
    Mat img_keypoints_1; Mat img_keypoints_2;
    drawKeypoints(img_1, keypoints_1, img_keypoints_1, Scalar::all(-1), DrawMatchesFlags::DEFAULT);
    drawKeypoints(img_2, keypoints_2, img_keypoints_2, Scalar::all(-1), DrawMatchesFlags::DEFAULT);
    //-- Show detected (drawn) keypoints
    imshow("Keypoints 1", img_keypoints_1);
    imshow("Keypoints 2", img_keypoints_2);
    waitKey(0);
    return 0;
}
/** @function readme */
void readme()
{
    std::cout << " Usage: ./SURF_detector <img1> <img2>" << std::endl;
}