Writing my own gaussian filter

I am aware there iseasier ways to do this using matlabs inbuilt functionality but I am trying to implement a Gaussian filter. I have this code so far but it is not working and the filtered image is coming out black. Any ideas, thank you!

 sig = 1.5;
[M,N] = size(img);
y = zeros(size(img));

r = 1; % Adjust for desired window size

for n = 1+r:N-r

 for m = 1+r:M-r

% Extract a window of size (2r+1)x(2r+1) around (m,n)

 w = img(m+(-r:r),n+(-r:r));

        GAUSS = 1/(2*pi*sig^2)*exp(-(M.^2+N.^2)/(2*sig^2));

GAUSS = GAUSS./sum(GAUSS(:));

        y(M,N) = GAUSS;

    end

end