Convert image to binary using openCV. Error: Assertion failed (scn==3 || scn==4)

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My simple code is as follows:

＃include "iostream"

＃include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

＃define TRACKBAR_MAX_VALUE  255
＃define THRESHOLD_MAX_VALUE 255 

Mat sourceImage;
Mat grayImage;
Mat binaryImage;

String windowNameBinarization = "Binarization";

int levels = 128;   

void binarization( void )
{
    cvtColor( sourceImage, grayImage, CV_BGR2GRAY );
    threshold( grayImage, binaryImage, levels, THRESHOLD_MAX_VALUE, THRESH_BINARY );
    imshow( windowNameBinarization, binaryImage );
}

void on_change ( int pos, void* )
{
    binarization();
}

int main( int argc, char **argv )
{
    String trackbarNameThreshold = "Threshold";
    sourceImage = imread("C:\2.jpg", 1);
    namedWindow( windowNameBinarization );
    createTrackbar( trackbarNameThreshold,  windowNameBinarization, &levels, TRACKBAR_MAX_VALUE, on_change );

    binarization();
    waitKey( 0 );

    return 0;
}