Is this a known problem? Is there a known solution?

I have a page with 2 ActionLinks - the first link updates the whole
page; the second link specifies a zone.

If you click on the two links in quick succession then an exception is
thrown. Alternatively, just Reload the page and click on the second
link before the page has finished reloading and you'll get the same
exception.

Here's the page, with source: http://202.177.217.122:8080/jumpstart/examples/javascript/ajax

Here's the exception:

A component event handler method returned the value
org.apache.tapestry5.internal.structure.BlockImpl@(protected)
org.apache.tapestry5.internal.structure.BlockImpl can not be handled.
Configured return types are java.lang.Class, java.lang.String,
java.net.URL, org.apache.tapestry5.Link,
org.apache.tapestry5.StreamResponse,
org.apache.tapestry5.runtime.Component.

Geoff