fillPoly crashes

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I have called

 std::vector< cv::Point > contour = contourIn; // initialization with input contour
 cv::Mat maskC(cv::Size(680, 480), CV_8UC1, cv::Scalar::all(0));
 cv::fillPoly(mask, contour, cv::Scalar::all(255));

fillPoly is crashing saying:

OpenCV Error: Assertion failed (i < 0) in getMat, file /home/stefan/git_repos/opencv/modules/core/src/matrix.cpp, line 963

Before I was having the same code, but I was calling the fillConvexPoly instead of fillPoly:

cv::Mat maskC(cv::Size(680, 480), CV_8UC1, cv::Scalar::all(0));
std::vector< cv::Point > contourCvx = convexingContour(contourIn); // initialization with the convex hull of the input contour
cv::fillConvexPoly(maskC, contourCvx, cv::Scalar::all(255));

I am using this to create a mask of the contours interior. I have changed to fillPoly because the input contour is not always convex.

Please help.