i divided images into 8x8 blocks using mat2cell,but it shows error .

my code is

y=[f1 f2 f3 f4 f5];
for li=1:length(y)
   div=[8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 ];
   b(li)=mat2cell(li(y),div,div);
end