Getting error Assertion failed.

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hello,

I am trying to communicate with simple camera of my laptop, but some time this code work fine and some time I am getting error.

#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

#include <iostream>
using namespace cv;
using namespace std;

int main() {
VideoCapture stream1(0);   //0 is the id of video device.0 if you have only one camera.
stream1.set(CV_CAP_PROP_FRAME_WIDTH, 640);
stream1.set(CV_CAP_PROP_FRAME_HEIGHT, 480);
if (!stream1.isOpened()) { //check if video device has been initialised
cout << "cannot open camera";
}

//unconditional loop
while (true) {
Mat cameraFrame;
stream1.read(cameraFrame);
imshow("cam", cameraFrame);
Mat roi = cameraFrame( Rect(150,50,150,250) );
imshow("cam1",roi);

if (waitKey(30) >= 0)
break;
}
return 0;
}

is there any suggestion to overcome this error.

Thank you very much in advance.