I want to use a floating point matrix (2d) or a 2d array to make a simple basic image (be it square or rectangle) and then convert it to grayscale and add poisson noise to it. But the code i have is not working.

 a = repmat(0:0.01:0.5,100,5);
 i = mat2gray(a);
j=imnoise(i,'poisson');
imshow(i)
figure();
imshow(j)

It shows the same image with and without poisson noise when i check the pixel value. Please help !