Derive a colour palette from an image

I have a question which asks to set the number of rows and columns (M and N) for the painting grid and resize an image to this required gridsize.

I used imresize like below, where B is my image.

B = imresize(B,[20,20]);

It then says use this command to derive the 8 colour palette.

[~, Palette] = kmeans(reshape(B(:),M*N,3),8,'E','s','S','U');

If i replace M and N with my grid values which are 20 and 20 i get this error, for the line above.

Error using  .* 
Integers can only be combined with integers of the same class, or scalar doubles.

Error in unifrnd (line 34)
r = mu + sig .* (2*rand(sizeOut)-1);

Error in kmeans (line 248)
            C = unifrnd(Xmins(ones(k,1),:), Xmaxs(ones(k,1),:));

Am i going about this the right way, and if so what is the meaning of the error message?

Thanks

Answer 1

[Image Analyst]

So did you try casting to double, like it hinted at, and try again?

[Andy]

Im not sure which value it is referring to but i tried casting all of them to double and got the same error message

[Image Analyst]

I don't have the stats toolbox so I can't replicate to help you.