libpng warning: Image width is zero in IHDR

edit

hello, I am trying to capture imgae from camera, using code

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>

using namespace cv;
using namespace std;

int main(int, char**)
{


    VideoCapture cap(0); // open the default camera
    cap.set(CV_CAP_PROP_FRAME_WIDTH, 640);
    cap.set(CV_CAP_PROP_FRAME_HEIGHT, 480);

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

    for(;;)
    {
        Mat frame;

        cap >> frame; // get a new frame from camera

        imwrite("C:/Users/ankit/Desktop/12.png", frame);

    }
    // the camera will be deinitialized automatically in VideoCapture destructor
    return 0;
}

but i am getting error like,

libpng warning: Image width is zero in IHDR
libpng warning: Image height is zero in IHDR
libpng error: Invalid IHDR data

please give me any suggesitions to solve this error.

Thank you very much in advace.