Vector subscript Out of Range Exception

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I am getting a vector subscript out of range exception, and a breakpoint is triggered at line 53. It is saving only 3 images out of all images being processed. How to solve this problem??

#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <iostream>
#include "opencv\cv.h"
#include <vector>

using namespace cv;
using namespace std;

vector<String> files;

int main()
{
int largest_area = 0;
int largest_contour_index = 0;
Rect bounding_rect;



    cv::glob("C:/Proj/Folder/5.Morph/Goal Keeping/1. Low balls/*.jpg", files);
for (size_t i = 0; i < files.size(); i++) {
    Mat src = imread(files[i]);

    Mat thr(src.rows, src.cols, CV_8UC1);
    Mat dst(src.rows, src.cols, CV_8UC1, Scalar::all(0));
    cvtColor(src, thr, CV_BGR2GRAY); //Convert to gray
    threshold(thr, thr, 25, 255, THRESH_BINARY); //Threshold the gray

    vector<vector<Point>> contours; // Vector for storing contour
    vector<Vec4i> hierarchy;

    findContours(thr, contours, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE); // Find the      contours in the image

    for (int j = 0; j< contours.size(); j++) // iterate through each contour. 
    {
        double a = contourArea(contours[j], false);  //  Find the area of contour
        if (a>largest_area) {
            largest_area = a;
            largest_contour_index = j;                //Store the index of largest contour
            bounding_rect = boundingRect(contours[j]); // Find the bounding rectangle for biggest     contour
        }

    }


    Scalar color(255, 255, 255);
    ***LINE 53*** drawContours(dst, contours, largest_contour_index, color, CV_FILLED, 8, hierarchy); // Draw the largest contour using previously stored index.
    rectangle(src, bounding_rect, Scalar(0, 255, 0), 1, 8, 0);
    imshow("src", src);
    imshow("Largest Contour", dst);

    // display result
    std::stringstream files(std::stringstream::in | std::stringstream::out);

    files << "Contour" << i << ".jpg";
    std::cout << "writing " << files.str().c_str() << " to disk" << std::endl;


    imwrite(files.str().c_str(), dst);

}


waitKey(0);
return 0;
    }