c++ openCv kMeans Assertion failed

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am running openCv kMeans on a vector of centre-points of contours. Like this:

      /// Load source image
     src = imread("image.jpg", 1);

         /// Convert image to gray and blur it
         cvtColor(src, src_gray, CV_BGR2GRAY);
          blur(src_gray, src_gray, Size(3, 3));
/// Detect edges using Threshold
    threshold(src_gray, threshold_output, thresh, 255, THRESH_BINARY);
    /// Find contours
    findContours(threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));

    /// Get the moments
    vector<Moments> mu(contours.size());
    for (int i = 0; i < contours.size(); i++)
    {
        mu[i] = moments(contours[i], false);
    }

    ///  Get the mass centers:
    vector<cv::Point2f> centroids(contours.size());

    for (int i = 0; i < contours.size(); i++)
    {
        centroids[i] = Point2f(mu[i].m10 / mu[i].m00, mu[i].m01 / mu[i].m00);           

    }


    int cNumber = sqrt(centroids.size() / 2);
    vector<Point2f> markerRawCorners(cNumber);

    //get clusters
    //FAILS BETWEEN HERE
    Mat labels, centers;
    int  attempts = 50, flags = cv::KMEANS_PP_CENTERS;
    TermCriteria tc;        

    if (centroids.size() > 5)
      {           
            kmeans(centroids, cNumber, labels, tc, attempts, flags, centers);   // points will get cast implicitly to Mat

    }
    //AND HERE

on some images, it works fine, but if i adjust the threshold, or use a different image, it gives me two Assertion failed errors, as below:

OpenCV Error: Assertion failed (y == 0 || (data && dims >= 1 && (unsigned)y < (unsigned)size.p[0])) in cv::Mat::ptr, file C:\OpenCV\opencv\sources\modules\core\include\opencv2/core/mat.hpp, line 429

OpenCV Error: Assertion failed (udata < (uchar)ptr && ((uchar)ptr - udata) <=(ptrdiff_t)(sizeof(void*)+16)) in cv::fastFree, file src\alloc.cpp, line 78

What do these errors mean? And how can i stop them happening?