matlab matrices error for image analysis

Trying to apply low pass filter to a black and white checkerboard following FFT, but don't understand why I continue to get this matrices error? Thank you in advance for your help!

a=imread(‘checkerboardbwlarge.jpg’); %Image is 276 x 276
af=fftshift(fft2(a)); 
figure,imshow(af); 
fftshow(af);
[x,y]=meshgrid(-138:137,-138:137); 
z=sqrt(x.^2+y.^2);
c=z<15 
af1=af.*c;

Error using  .* 
Matrix dimensions must agree.

Answer 1

[Walter Roberson]

.jpg images are never 2 dimensional; .jpg always uses RGB. You need

af=fftshift(fft2(a(:,:,1)));

[NiI133]

the image is a checkerboard so i assumed I did not have to change it to grayscale. Will try this and see if it works. Thank you!