Why do I get subscript indices error?

In the rest of the code I used a similar method,

r3 = randi([1 N],[G 1]);
for i = 1:G
  ch = ceil(r3(i,1)/3840);
  [r, c] = ind2sub([4 960],r3-((ch-1)*3840));
  if Ma{ch,1}(r,c) == 0
      Ma{ch,1}(r,c) = 1;
  else
      Ma{ch,1}(r,c) = 0;
  end
end

This time 'r' and 'c' can't be zero, but I get the same error here! Would you please help me with this too?