stores cv::Mat into a std::vector

edit

i'am trying to write many functions computing gradient with deferent kernels which return a vector of Mat object here one exemple :

std::vector<cv::Mat> histogram::SobelDirections(cv::Mat &frame){
    Mat vectMat;
    Mat Sx;
    cv::Sobel(frame, Sx, CV_32F, 1, 0, 3);

    Mat Sy;
    cv::Sobel(frame, Sy, CV_32F, 0, 1, 3);

    imshow("x", mat2gray(Sx));
    imshow("y", mat2gray(Sy));

    vectMat.push_back(Sx);
    vectMat.push_back(Sy);

    return vectMat;

}

i get this , any help would be appreciated

Program:  C Windows system321MSVCP110D.  dll 
File:  C:\Program Files icrosoft Visual Studio Include Vector Line:  1124
Expression: vector subscript out of range

here main code

    cv::Mat mat2gray(const cv::Mat& src)
    {
        Mat dst;
        cv::normalize(src, dst, 0.0, 255.0, cv::NORM_MINMAX, CV_8U);

        return dst;
    } 

    int main(int argc, char* argv[])
    {
        Mat image =Mat::zeros(Size(320, 240), CV_8UC1);
        circle(image, Point(160, 120), 80, Scalar(255, 255, 255), -1, CV_AA);
        imshow("x", mat2gray(SobelDirections(frame).at(0));
        imshow("y", mat2gray(SobelDirections(frame).at(1));
        return 0;
    }