What am I doing wrong with HoG implementation?

I'm trying to implement HoG and I'm not sure what I'm doing wrong when finding the cell's histogram.

function oim = HoG(im)

cellsize = 3;

if(~isa(im,'double'))
    im = im2double(im);
end

kernel = [-1 0 1];
%Computing gradients
gx = imfilter(im,kernel);
gy = imfilter(im,kernel');
mag = (gx.^2 + gy.^2).^0.5;
%Finding orientations of the gradients and making it unsigned
ang = (180*atan2(gy,gx))/pi;
ang(ang<0) = ang(ang<0) + 180;

%Finding the orientation histogram of the first cell
tempA = reshape(ang(1:cellsize,1:cellsize),[cellsize*cellsize 1]);
tempM = reshape(mag(1:cellsize,1:cellsize),[cellsize*cellsize 1]);

histC = zeros(9,1);
lowerIndex = floor(tempA./20);
upperIndex = lowerIndex + 1;
upperCenter = (lowerIndex + 1.5)*20;
lowerCenter = upperCenter - 20;
upperIndex(upperIndex == 9) = 0;

%Finding constributions to upper and lower bins
contriU = (tempM.*(upperCenter-tempA)./20);
contriL = (tempM.*(tempA - lowerCenter)./20);

for i = 0:8
    histC(i+1,1) = histC(i+1,1) + sum(contriL(lowerIndex == i));
    histC(i+1,1) = histC(i+1,1) + sum(contriU(upperIndex == i));
end
oim = histC;

end

The code is written upto Orientation binning part and I'm getting negative values in the histogram.

Thank you.