[Seaside] lightbox - used in a callback - what am I doing wrong?

[Squeaker]

Greetings,

I have a page with an IMG on it and I would like to get a lightbox when 
the user clicks on the IMG.  The following test code does not work. I 
know that the callback is invoked (using breakpoints) but the lightbox 
is not displayed. Can you tell me what I am doing wrong?

--- start of code ---

renderContentOn: html

   html div id: 'image';
     with: [html image url: 'http://localhost/resources/foo.png';
                 onClick: (html evaluator callback: [:script |
                           self lightbox:
                           (SUClassBrowser new newClass: MyTestClass)];
                 return: false)]

--- end of code ---

Thanks,
Frank