Debug assertion fail when using grayscale

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I am trying to make a hand recognition system but when i used grayscale for cvtColor, i get debug assertion fail but when i use HSV the code works fine. Can you resolve this ? I am a newbie in opencv.

#include "stdafx.h"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/objdetect.hpp"
#include < opencv2\opencv.hpp>    
#include < stdio.h>  
#include <iostream>


using namespace std;
using namespace cv;

int thresh = 100;


int findBiggestContour(vector<vector<Point> > contours){
    int indexOfBiggestContour = -1;
    int sizeOfBiggestContour = 0;
    for (int i = 0; i < contours.size(); i++){
        if (contours[i].size() > sizeOfBiggestContour){
            sizeOfBiggestContour = contours[i].size();
            indexOfBiggestContour = i;
        }
    }
    return indexOfBiggestContour;
}

void shifcontour(vector<Point>& contour, int x, int y)
{
    for (size_t i = 0; i<contour.size(); i++)
    {
        contour[i].x += x;
        contour[i].y += y;
    }
}
int main()
{
    cout << "beginning";

    VideoCapture cap("pathaka.MP4");

    if (!cap.isOpened())  // check if we succeeded
        return -1;

Ptr<BackgroundSubtractor> pMOG2 = createBackgroundSubtractorMOG2();
    for (;;)
    {
        Mat original, img;
        cap >> img;
        imshow("Source", img);

        Mat hsv;
        cvtColor(img, hsv, CV_BGR2GRAY);

        Mat bw;
        inRange(hsv, Scalar(0, 30, 80), Scalar(20, 150, 255), bw);
        GaussianBlur(bw, bw, Size(7, 7), 1.5, 1.5);
        Canny(bw, bw, 0, 30, 3);


        vector<vector<Point> > contours;
        vector<vector<Point> > convex_hull;
        vector<Vec4i> hierarchy;


        int erosion_type = MORPH_ELLIPSE;
        int erosion_size = 0;
        Mat element = getStructuringElement(erosion_type,
            Size(2 * erosion_size + 1, 2 * erosion_size + 1),
            Point(erosion_size, erosion_size));

        dilate(bw, bw, element);

        findContours(bw, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));

        int s = findBiggestContour(contours);

        Mat drawing = Mat::zeros(img.size(), CV_8UC1);

        dilate(drawing, drawing, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));
        dilate(drawing, drawing, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));

        std::vector<cv::Point> cnt;
        cnt = contours[s];

        Moments M;
        M = cv::moments(cnt);

        cv::Point result;
        result = cv::Point(M.m10 / M.m00, M.m01 / M.m00);

        Point center(drawing.cols / 2, drawing.rows / 2);

        cv::circle(drawing, center, 3, Scalar(255, 255, 255), -1, 8, 0);


        int x;
        if (result.x > center.x)
        {
            x = result.x - center.x;
            x = -x;
        }
        else
        {
            x = result.x - center.x;
        }

        int y;

        if (result.y < center.y)
        {
            y = center.y - result.y;
        }
        else
        {
            y = center.y - result.y;
        }

        cout << "x:" << x << endl;
        cout << "y: " << y << endl;


        shifcontour(contours[s], x, y);

        drawContours(drawing, contours, s, Scalar(255), -1, 8, hierarchy, 0, Point());

        imshow("Hsv", drawing);
        if (waitKey(30) >= 0) break;
    }

    return 0;
}