I'm trying to display multiple images using subplot but Im getting "add axes to grid" error....How do I rectify it???

subplot(2,2,1);imshow(a);title('a');

subplot(2,2,2);imshow(b);title(b'');

subplot(2,2,3);imshow(c);title(c'');

subplot(2,2,4);imshow(d);title('d');

[Walter Roberson]

Please post a copy of the complete error message.