It looks like you are using RPC instead of JAXB to convert from java objects to xml and back, are you just using a web service plugin for eclipse? You may have to update how you are generating the classes. In any event what's going on is that java and xml both have types and when you convert between java objects and xml (which it has to for these services) it has certain classes that map to certain xml types, and visa versa. It looks like whatever method you are using doesn't have a mapping for the Object class. I've never used the Eclipse plugin for generating the classes, but there are plenty of command line tools that you can use if you can't get the plugin figured out, I use wsconsume for the most part. Hope this helped!