Finding homography matrix

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What is wrong in my code? It gives bad homography matrix for some reason.

SurfFeaturesFinder finder(1);
    vector<ImageFeatures> vec_features;
    for(int i=0;i<vec_im.size();++i)
    {
        ImageFeatures t;
        finder(vec_im[i],t);
        t.img_idx = i;
        vec_features.push_back(t);
    }
    finder.collectGarbage();
int k=0;
    for(int i=0;i<vec_features.size();++i)
    {
        ++k;
        for(int j=k;j<vec_features.size();++j)
        {
            Ptr<DescriptorMatcher> matcher = new BFMatcher(NORM_L2);
            vector<Point2f> prev_keys, curr_keys;
            std::vector<DMatch> matches;
            matcher->match(vec_features[i].descriptors, vec_features[j].descriptors, matches);
            for (int p = 0; p < (int)matches.size(); ++p)
            {
                prev_keys.push_back(vec_features[i].keypoints[matches[p].queryIdx].pt);
                curr_keys.push_back(vec_features[j].keypoints[matches[p].trainIdx].pt);
            }
            Mat M = findHomography(prev_keys, curr_keys, CV_RANSAC);
            cout<<M<<" "<<M.rows<<"x"<<M.cols<<endl;
            cout<<M(Rect(0,0,3,2))<<endl;
            Mat im= Mat::zeros(vec_im[j].rows,vec_im[j].cols,vec_im[j].type());
            warpAffine(vec_im[j], im, M(Rect(0,0,3,2)), im.size());
            imshow("im", im);
            Mat merged = Mat::zeros(vec_im[i].rows,vec_im[i].cols,vec_im[i].type());
            addWeighted(vec_im[i], 0.5, im, 0.5, 0.0, merged); 
            imshow("out", merged);
            cvWaitKey(0);
        }
    }