Why am I getting an error when I use grayscale images with

% Step 1 - Load grayscale image
imGray = imread('grayscaleImage.tif');

% Step 2 - Replicate image into RGB layers using REPMAT
nRV = 1; % Number Of Replications Vertically
nRH = 1; % Number Of Replications Horizontally
nRL = 3; % Number Of Replication Layers
imRGB  = repmat(imGray,[nRV, nRH, nRL]); % Replicate grayscale image 3 times

% Step 3 - Write resized image to file
imageName = 'rgbImage.tif';
imwrite(imRGB,imageName)

% Step 4 - Generate HDR from TIF File
imageNames = {imageName}; % MAKEHDR uses a cell array of strings as input.
RE = 1; % Currently a scalar, but would be a matrix for multiple images.
imHDR = makehdr(imageNames,'RelativeExposure',RE); % Create HDR image.

% Step 5 - Display image
imLDR = tonemap(imHDR); % Create LDR image from HDR image for display purposes.
figure; imshow(imLDR) % Display LDR image.

2. Use CAT to concatenate the 2D image 3 times

% Step 2 - Concatenate image into RGB layers using CAT
nRL = 3; % Number Of Replication Layers
imRGB = cat(nRL,grayImage,grayImage,grayImage);