Segmentation fault during MapperGradAffine.calculate() [closed]

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The following is a snippet of code from opencv/opencv_contrib/module/reg/src/mapper.cpp. If img.type() in the create member is CV_8U, then the Mat created cannot contain any double value which causes a segmentation fault. Am I right here and if so what can I do to change it?

 void Mapper::grid(const Mat& img, Mat& grid_r, Mat& grid_c) const
{
    // Matrices with reference frame coordinates
    grid_r.create(img.size(), img.type());
    grid_c.create(img.size(), img.type());
    if(img.channels() == 1) {
        for(int r_i = 0; r_i < img.rows; ++r_i) {
            for(int c_i = 0; c_i < img.cols; ++c_i) {
                grid_r.at<double>(r_i, c_i) = r_i;
                grid_c.at<double>(r_i, c_i) = c_i;
            }
        }
    } else {
        Vec3d ones(1., 1., 1.);
        for(int r_i = 0; r_i < img.rows; ++r_i) {
            for(int c_i = 0; c_i < img.cols; ++c_i) {
                grid_r.at<Vec3d>(r_i, c_i) = r_i*ones;
                grid_c.at<Vec3d>(r_i, c_i) = c_i*ones;
            }
        }
    }
}