Apparently the error is caused by a class name clash. The @Entity ProductImpl implements a.b.c.Product a.b.c.Product can be found in 2 jars inside the classpath. As soon as I remove the duplicated interface the error goes away. Now I know this is bad form to have class name clashes but the error was not occurring in jboss 4.0.5. How do you propose to handle this error? At the very least we should get a more informative error message to help troubleshoot the issue. I lost a day on this. Anyway I have a testcase that reproduces the problem, where can I submit it ? Thanks, -Guillaume View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4132243#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...