Modify cpselect images from its handle... linking \ listener error?

I'm trying to modify one of the images in cpselect in the following manner:

im1 = imread('ngc6543a.jpg');
im2 = imread('ngc6543a.jpg');
im3 = imread('ngc6543a.jpg');
im3(100:200,100:200) = 0;
hFig = cpselect(im1,im2);
hax = findobj(hFig,'Type','axes');
set(findobj(hax(1),'Type','image'), 'CData', im3);

This results in an error... any ideas?