Why does dilate and erode images produce black images?

Did i do something wrong with nhood variable?

%% 1. Dilation and Erosion
%a. Apply dilation and erosion to “lung.jpg” using 3x3 structuring element (all pixel value is 1). 
%Before applying dilation and erosion you must perform the thersholding to make the input image into binary one (threshold = 128).
lung = imread('lung.jpg');
bilung = imbinarize(lung,128); 
nhood = strel(ones(3,3));
dilatedlung = imdilate(bilung,nhood);
erodedlung = imerode(bilung,nhood); 

%b. Display original image, dilation image and erosion image within the same figure.
figure
subplot(1,3,1);
imshow(lung);
title('original img');
subplot(1,3,2);
imshow(mat2gray(dilatedlung));
title('dilated img');
subplot(1,3,3);
imshow(mat2gray(erodedlung));
title('eroded img');

%c. Get morphological gradient of “lung.jpg” and display it. 
basic_gradient = dilatedlung - erodedlung;

figure
imshow(basic_gradient);
title('gradient');

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[Image Analyst]

Try

basic_gradient = imabsdiff(dilatedlung, erodedlung);

Answer 2

[Image Analyst]

You didn't use imbinarize correctly. Get rid of the 128.

%b. Display original image, dilation image and erosion image within the same figure.
figure
subplot(1,3,1);
imshow(lung);
title('original img');
subplot(1,3,2);
imshow(mat2gray(dilatedlung));
title('dilated img');
subplot(1,3,3);
imshow(mat2gray(erodedlung));
title('eroded img');
%c. Get morphological gradient of “lung.jpg” and display it. 
basic_gradient = imabsdiff(dilatedlung , erodedlung);
figure
imshow(basic_gradient);
title('gradient');