Store cv::Mat in Swift variable

I have a c++ opencv function that gets the gray scale of an image and stores it into an array.

+(cv::Mat) makeGray:(UIImage *) image {
    // Transform UIImage to cv::Mat
    cv::Mat imageMat;
    UIImageToMat(image, imageMat);

    //Transform image to grayscale
    cv::Mat grayMat;
    cv::cvtColor(imageMat, grayMat, CV_BGR2GRAY);
    return grayMat;
}

Whenever I try storing this in a variable using swift

swift function:

@IBAction func convertImage(_ sender: UIButton) {
    let grayMat = OpenCVWrapper.makeGray(photoImageView.image)
}

The entire program just freezes without throwing out an error.

The swift function works great when I return an UIImage:

+(UIImage *) makeGray:(UIImage *) image {
    // Transform UIImage to cv::Mat
    cv::Mat imageMat;
    UIImageToMat(image, imageMat);

    //Transform image to grayscale
    cv::Mat grayMat;
    cv::cvtColor(imageMat, grayMat, CV_BGR2GRAY);
    return MatToUIImage(grayMat);
}

But I'm looking to return a 3darray and not an Image.