关于matlab的图像相减

i=imread('1.jpg')
j=imread('2.jpg')
i=double(i)/255;
j=double(j)/255;
x=i-j;
imshow(x);
x=i-j;
就是不能运行,显示??? Error using ==> image
TrueColor CData contains element out of range 0.0 <= value <= 1.0.

Error in ==> F:\MATLAB6p5\toolbox\images\images\imshow.m
On line 104 ==> hh = image(xdata, ydata, cdata, 'BusyAction', 'cancel', ...
该怎么改呀?而且我两幅图像的大小是一样的!!1024*768:

望指点,如何实现任意两幅图像的相减!!!!