OpenCV(4.1.0) Error: Assertion failed (((0x28442211 >> ((traits::Depth<_Tp>::value) & ((1 << 3) - 1))*4) & 15) == elemSize1()) in cv::Mat::at, file c:\opencv\build\include\opencv2\core\mat.inl.hpp, line 1145

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I used this code in Visual Studio 2017 to convert the 16-bit grayscale image to 8-bit, but the following question appeared when I started executing the exe file. ##

#include<iostream>
#include <opencv2/opencv.hpp>
#include <string>
#include <stdio.h>
//#include "pch.h"
//#include <direct.h>
using namespace std;
using namespace cv;
int main(void) {
 char buff1[100];
 char buff2[100];
 for (int i = 1; i < 21; i++) {
    //sprintf_s(buff1, "E:\\Mask_RCNN-master\\My Images\\labelme_json\\%d_json\\label.png",i);
    //sprintf_s(buff2, "E:\\Mask_RCNN-master\\My Images\\cv2_mask\\%d.png", i);
    sprintf_s(buff1, "./My Images/labelme_json/%d_json/label.png", i);
    sprintf_s(buff2, "./My Images/cv2_mask/%d.png", i);
    //sprintf(buff1,"/media/lj/FA68-10A6/test_drug/disp/disp_%d.png",i);
    //sprintf(buff2,"/media/lj/FA68-10A6/test_drug/disp_8/disp_%d.png",i);
    //Mat src;
    //Mat dst;
    Mat src = imread(buff1, IMREAD_UNCHANGED);
    Mat ff = Mat::zeros(src.rows, src.cols, CV_8UC1);
    for(int k = 0; k<src.rows; k++)
    {
        for (int kk=0; kk<src.cols; kk++)
        {
            int n = src.at<ushort>(k, kk);
            ff.at<uchar>(k, kk) = n;
        }
    }
    imwrite(buff2,ff);
}
return 0;
}

I get the error: ##

• OpenCV(4.1.0) Error: Assertion failed (((0x28442211 >> ((traits::Depth<_Tp>::value) & ((1 << 3) - 1))*4) & 15) == elemSize1()) in cv::Mat::at, file c:\opencv\build\include\opencv2\core\mat.inl.hpp, line 1145
• OpenCV: terminate handler is called! The last OpenCV error is: OpenCV(4.1.0) Error: Assertion failed (((0x28442211 >> ((traits::Depth<_Tp>::value) & ((1 << 3) - 1))*4) & 15) == elemSize1()) in cv::Mat::at, file c:\opencv\build\include\opencv2\core\mat.inl.hpp, line 1145

I hope to get your advice. Thank you