Error: Assertion failed (size.width>0 && size.height>0) in imshow

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Hi all,

Just installed OpenCV yesterday and I'm trying to play around with it, to eventually work on some fun projects. At the moment I'm struggling to visualize my laptop webcam, and I can't figure out why. This is the code I'm running:

int main() {
    VideoCapture stream1(0);

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

    return 0;
 }

After building the project and running it, this is the error that comes up:

OpenCV(3.4.1) Error: Assertion failed (size.width>0 && size.height>0) in imshow

Thanks in advance for your help ! :)