Program crashes when exiting function

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I have this function that crashes on exit and I cant seem to figure out why. I hope it isn't a problem with the opencv or visual studio version because I have many other code in the program thats calling opencv functions and it runs fine. The only problem is in this function.

void compute_hog(const vector< cv::Mat > & img_lst, vector< cv::Mat > & gradient_lst, const cv::Size & size)
{
    cv::HOGDescriptor hog;
    hog.winSize = size;
    cv::Mat gray;
    vector< cv::Point > location;
    vector< float > descriptors;

    vector< cv::Mat >::const_iterator img = img_lst.begin();
    vector< cv::Mat >::const_iterator end = img_lst.end();
    //for (; img != end; ++img)
    //{
        cv::cvtColor(*img, gray, cv::COLOR_BGR2GRAY);
        hog.compute(gray, descriptors, cv::Size(8, 8), cv::Size(0, 0), location);
        gradient_lst.push_back(cv::Mat(descriptors).clone());
        //get_hogdescriptor_visu(img->clone(), descriptors, size);

        //#ifdef _DEBUG
        //cv::imshow("gradient", get_hogdescriptor_visu(img_lst[0], descriptors, size));
        //cv::waitKey(1);
        //cv::destroyAllWindows();

        //#endif
    //}
}