unable to perform assignment error in im2col

Hello, I think am making a basic error somewhere but I can not figure it out yet. How best can I achieve this, or what type of image is im2col expecting here? I tried indexed version to no avail.

Here is the minimal code that reproduces the error

im = imread('Image.jpg');
im = im2double(im);
im2col(im, [8 8], 'distinct');

where Image.jpg is of type uint8.

the error i get:

Unable to perform assignment because the size of the left side is 420-by-560 and the
size of the right side is 420-by-560-by-3.

Error in im2col (line 63)
        aPad(1:size(a,1),1:size(a,2)) = a;

Thank you so much

Answer 1

[Walter Roberson]

https://www.mathworks.com/help/images/ref/im2col.html#d122e88473

A

Image, specified as a 2-D grayscale image, 2-D binary image, or 2-D indexed image.

[MatlabEnthusiast]

however, I missed that at this hour only God knows. thanks.