cv2 findChessboardCorners does not detect corners

I want to try out this tutorial and therefore used the code from here in order to calibrate my camera. I use this image:

The only thing I adapted was chessboard_size = (14,9) so that it matches the corners of my image. I don't know what I do wrong. I tried multiple chessboard pattern and cameras but still cv2.findChessboardCorners always fails detecting corners. Any help would be highly appreciated.

Finally I could do it. I had to set chessboard_size = (12,7) then it worked. I had to count the internal number of horizontal and vertical corners.