Getting error -213: Unsupported combination of source format (=0), and destination format (=4) in function getLinearFilter

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I am trying to apply the inbuilt laplacian operator on an image but I am getting error "error: (-213) Unsupported combination of source format (=0), and destination format (=4) in function getLinearFilter" Please tell how to resolve. Complete code:

#include "opencv2/opencv.hpp"
#include "opencv2/imgproc/imgproc.hpp"

#include <iostream>
#include <cmath>
#include <vector>
#include <string>

using namespace std;
using namespace cv;

const char ImageFilePath[] = "/home/user/Downloads/skeleton.tif";

/**
 * @function main
 */

int main(int argc, char** argv) {
    Mat src, dst;
    Mat gray;

    //

    /// Load image

    src = imread(ImageFilePath, 1);

    //gray = Mat(src.rows, src.cols, CV_32SC1);

    if (!src.data) {
        return -1;
    }

    //cout << src.channels() << endl;
    cvtColor(src, gray, CV_BGR2GRAY);
    gray.convertTo(gray, CV_32SC1);

    Mat res, abs_dst;
    //res = gray.clone();

    namedWindow("src image", CV_WINDOW_AUTOSIZE);
    imshow("src image", gray);

    Laplacian(gray, res, CV_32SC1, 3, 1, 0, BORDER_DEFAULT);
    //namedWindow("Laplacian", CV_WINDOW_AUTOSIZE );
    //imshow("Laplacian", res );

    res += gray;

    //convertScaleAbs( res, abs_dst );
    //namedWindow("Addition", CV_WINDOW_AUTOSIZE );
    // imshow("Addition", abs_dst );

    waitKey(0);
    return 0;

}