Help: Error in my code PSNR

I am using this code to and calculating PSNR values even if I am not embedding even a single bit, still the PSNR value i am getting is 10

clc;
clear all;
fprintf('The image matrix A-');
fprintf('\n');
A=imread('airplane.bmp');
figure(1), imshow(A); title('Original Image (Cover Image)');
[Z,map]=rgb2ind(A,256);
p=size(Z);
disp(p);
B=Z;
B(2:2:end,:) = fliplr(Z(2:2:end,:));
C=reshape(B',1,[]);
disp(Z)
fprintf('Inverse S-scan of A-');
fprintf('\n');
disp(C);
fprintf('Size Of Inverse S-scan Matrix-');
fprintf('\n');
z=size(C);
t=z(1)*z(2);
disp(t);
for n=1:1:t
  if (n==1)
     D(n)=C(1,1);
  elseif n>1
      D(n)=C(1,(n-1))-C(1,n);
  end
end

for n=1:1:t if (n==1) E(n)=D(1,1); else if(D(n)>0) E(n)=D(1,n)+1; else E(n)=D(1,n); end end end

for n=1:1:t if(n<2) F(n)=E(1,n)+1; else F(n)=E(n); end end

for n=1:1:t if(n==1) G(n)=F(1,1); else G(n)=C(1,n-1)-F(1,n); end end

M=reshape(G,p(1),p(2)); M(:,2:2:end)=flipud(M(:,2:2:end)); M=M.';

N=ind2rgb(M,map);

figure(2);imshow(N);title('Stego Image');

peaksnr = psnr(M,B) err = immse(M,B)