How can I get the histeq function to ignore NaN values?

% Setting up a sample image 
% Use im2double as double saves NaN while uint8 converts it to 0

I = im2double(imread('cameraman.tif'));

I(1:100,1:100) = NaN
I(1:256,1:100) = NaN
I(1:256,180:end) = NaN
I(1:100,:) = NaN
I(160:end,:) = NaN

figure, imshow(I)
% Duplicate Matrix
I2 = I;

% Store the output of the contrasted stretched image in I2
I2(~isnan(I2)) = histeq(I2(~isnan(I2)));
figure, imshow(I2)


J = histeq(I) transforms the grayscale image I so that the histogram of the output grayscale image J has 64 bins and is approximately flat.