fft2 on grayscale image

[Peter von Niederhaeusern]

Hi,

I'm a bit confused about the fact that I'm not able to fft2 a grayscale image.
I get the following error:

%%% snippet %%%

// color image, works
square=imread('square.bmp');
square_fft2=fftshift(fft2(square,246,247));

// grayscale image, does not work
square_g=RGB2Gray(square);
square_fft2=fftshift(fft2(square_g,246,247));

!--error 4
Undefined variable: x

at line      45 of function fft2 called by :
square_fft2=fftshift(fft2(square_g,246,247));

%%% snippet %%%

What's the reason/rationale for this error?

Thank you very much for a hint.

Peter