I do almost the same and it works for me. The only difference seems to be that I do not use an a4j:outputPanel but a normal one. So not sure if it is in your case facelets related or has something to do with a4 or maybe even richfaces.
Ronald, I've change the a4j:outputPanel to a h:panelGroup and still get the exception.
I think the problem is a little worst than that...