Why does my screen go black?

My region filled image and skeletonized image just appears black

I fill in all the bones using the function below

%% 3. Region Filling
%a. Region filling the “hand.jpg”. You should fill all bones.
hand = imread('hand.jpg');
n = imbinarize(hand); 
sq = ones(3,3);
nb=n&imerode(n,sq);
nf=regfill(nb,[227,178],sq);  %regfill fxn
                              %the centre of the image is [227,178] 

figure
subplot(1,3,1);
imshow(n) 
subplot(1,3,2); 
imshow(mat2gray(nb)) 
subplot(1,3,3);
imshow(mat2gray(nf))

%b. Make Skeletons of filled hand image. You can use proper structuring element.
nk=imskel(nf,sq); %imskel fxn
figure
imshow(nk) 

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[Image Analyst]

Why not simply use imfill()???

nFilled = imfill(n, 'holes');