a4j:commandButton cannot handle event if reRender a tree

[kingkii]

I have some code like this:

<h:form>
<rich:tree id="catalog_tree" ...>
...
</rich:tree>
</h:form>


<rich:modalPanel id="contentservice_panel">
<h:form>
...
<a4j:commandButton style="float:right" action="#{contentService.onFinish}" value=" Finish " reRender="#{contentService.reRender}" oncomplete="Richfaces.hideModalPanel('contentservice_panel')"/>
</h:form>
</rich:modalPanel>

If my backing bean #{contentService.reRender} returns "catalog_tree", the oncomplete event cannot be handled. But if the it returns some other object, the oncomplete works.
Is it a bug?