Collection which contains a to-be-ignored structure does not work correctly

I defined my own unmarshaller to translate code-elments like <code>DE</code> into existing objects read from my database. The problem occurs when i try to unmarshall a collection of such objects:

Binding:
<collection set-method="setNationalities" item-type="customizing.Country" factory="io.XMLBindingUtils.factorySet">
<structure name="citizenship">
<structure name="code" type="customizing.Country"
unmarshaller="io.CustomizingDataUnmarshaller"/>
</structure>
</collection>

XML-file snipet:
<citizenship>
<code>US</code>
</citizenship>
<citizenship>
<code>DE</code>
</citizenship>

This code should create a Set of Country objects and call setNationalities to store the set in my object.
However setNationaties is always called with an empty Set. If remove the tag <citizenship> from XML-file and from the binding it works (see below) ! Note, that my unmarshaller works correctly. It finds and returns Country-Objects as expected but the add-method of the Set is called with an java.lang.object instead of a customizing.Country object.

Binding:
<collection set-method="setNationalities" item-type="customizing.Country" factory="io.XMLBindingUtils.factorySet">
<structure name="code" type="customizing.Country"
unmarshaller="io.CustomizingDataUnmarshaller"/>
</collection>

XML-file snipet:
<code>US</code>
<code>DE</code>

This seems to be a bug. I hope you can tell me a workaround. Is there any other way to ignore the citizenship tag ?