Doesn't work with MultiHashMap

The following throws a NPE deep inside java.util.HashMap$HashIterator:

XStream xStream = new XStream();
MultiHashMap map = new MultiHashMap();
map.put("a", "stuff");
map.put("a", "more stuff");
Map output = (Map) xStream.fromXML(xStream.toXML(map));
Collection collection = output.values();
for (Iterator iterator = collection.iterator(); iterator.hasNext() {
Object o = (Object) iterator.next();
System.out.println("o = " + o);
}