ERR “Assertion `img_input.empty() == false' failed” in convert YUV to Grayscale

edit

I am trying to verify bgslibrary(background substraction algorithm in ) performance in my simple program. first, convert YUV422 to grayscale, it is just extract Y part from YUV and save it to memory of gray image:

void yuv_to_gray(void *yuv_ptr, void *gray_ptr, size_t size)  { 
    for (int i = 0; i!= size; i+=2) {
        *(((unsigned char *)gray_ptr)+i/2) = *(((unsigned char *)yuv_ptr)+1+i);
    }
}

then I constructed opencv MAT types, which used in process function, main part of the code shown as below:

    {
        void * gray;
        if ((gray = (void *)malloc(size/2*sizeof(unsigned char)))==NULL){
            cout << "malloc error" << endl;
            return 1;
        }
        memset(gray, 0, size/2*sizeof(unsigned char));
        yuv_to_gray(ptr, gray, size);
        Mat frame_in_gray(height, width, CV_8UC1, gray);
        Mat frame_out(height, width, CV_8UC1);
        Mat frame_bg(height, width, CV_8UC1);
        IBGS *fd;
        fd = new FrameDifference;
        clock_t start = clock();
        fd->process(frame_in_gray, frame_out, frame_bg);
        delete fd;
        free(gray);    
}

then start to run program, error reported as:

void bgslibrary::algorithms::IBGS::init(const cv::Mat&, cv::Mat&, cv::Mat&): Assertion `img_input.empty() == false' failed.

Concluded from the error, Mat frame_in_gray is empty , but I do use the correct construction function, and make its data link to *gray pointer. Any guys can give some suggestion ?