index exceeds matrix dim

I am working on 3d image encryption using 8 cross8 matrix. where image size is 256 256*3. portion of the code is attached. Can some one explain why i m gettting index excedes matrix dim

I1=imread('Hello.png')
h=size(I1);
:
:
EncImg2=zeros(h);
for z=1:7
for i=1:h(1)
    for j=1:8:h(2)
    P=double(EncImg(i,j:j+7,z));
    EncImg2(i,j:j+7,z)=mod((Km*P')',256);
    end
end

I am constantly getting 'index exceeds matrix dim ' at P. Can anyone explains whats wrong with this line and what to do

[KALYAN ACHARJYA]

Duplicate question:

https://in.mathworks.com/matlabcentral/answers/474309-loop-for-image-encryption-gives-error#answer_385710?s_tid=prof_contriblnk