C++ BLOB OPENCV的ROI断言失败

我有一个程序，尝试使用simpleblobdetector检测一个blob，在检测到blob后，我尝试对blob进行ROI，以便我能够知道blob的RGB值的平均值。但大多数情况下，在运行程序或移动对象（因为这是网络摄像头输入）后，我会出现以下错误：

在引发“cv:：Exception”的实例后调用terminate what（）：OpenCV（4.2.0）…/modules/core/src/matrix.cpp:465:错误：（-215:断言失败）0

    // filterByColor
    params.filterByColor = true;
    params.blobColor = 255;


    // Filter by Area.
    params.filterByArea = true;
    params.minArea = 4000;
    params.maxArea = 100000;

    // Filter by Circularity
    params.filterByCircularity = true;
    params.minCircularity = 0.45;

    // Filter by Convexity
    params.filterByConvexity = true;
    params.minConvexity = 0.45;

    // Filter by Inertia
    params.filterByInertia = true;
    params.minInertiaRatio = 0.01;

    // Storage for blobs
    vector<KeyPoint> keypoints;

    // Set up detector with params
    Ptr<SimpleBlobDetector> detector = SimpleBlobDetector::create(params);

    // Detect blobs
    detector->detect( frameBrightness, keypoints);


            Mat im_with_keypoints;
            cv::Scalar mean;
                drawKeypoints( frameBrightness, keypoints, im_with_keypoints, Scalar(0,0,255), DrawMatchesFlags::DRAW_RICH_KEYPOINTS );

                   size_t i;
                   Point Coordinate;
                    for( i =0; i < keypoints.size(); i++ )
                    {

                        Coordinate =    keypoints[i].pt ;
                    Rect r(Coordinate.x-(keypoints[i].size /2), Coordinate.y-(keypoints[i].size/2), keypoints[i].size, keypoints[i].size) ;
                    Mat roi(im_with_keypoints, r);
                    mean = cv::mean(roi);
                    qDebug() << "Blue" << mean[0] ;
                    qDebug() << "Green" << mean[1] ;
                    qDebug() << "Red" << mean[2] ;