java.util.Map values unexpectedly cast to java.lang.String

Hi everybody
while using elasticsearch.org I stumbled upon a weird error. The Scala objects stored into a java.util.Map are for some reason cast to java.lang.String, whereas Java objects seem to be fine. Elasticsearch's main developer is looking at the issue, but I suspect it could be some more or less well-known Java-Scala interoperability problem. Could anybody enlighten me?
This is the Scala code to replicate the problemhttps://gist.github.com/1332314#file_custom_score_query_builder_params.scalaand here is the stack tracehttps://gist.github.com/1332314#file_stacktrace.txt
Thanks!
CheersAlex