First of all don't use sum as a variable since it's a MATLAB function, next time you try to call the function it will be obfuscated by your variable.
Second img is a uint8 class which cannot store more than 256 values (zero included):
intmax('uint8')
Do the following
out = sum(double(img(:)))
Note that I am using the function sum to perform the summation and assignin the value to something else but any MATLAB function named variable.
