I am getting the same error. I am guessing it's a library issue but I have not been able to resolve it even after replacing libraries. Anyone have any ideas what the correct combination of libraries are for Hibernate 3.3.1 to work. I am using. JBoss 4.2.3 Javassist 3.4.GA Hibernate 3.3.1 08:07:39,746 ERROR [BasicLazyInitializer] Javassist Enhancement failed: com.comanche.web.dao.beans.Source java.lang.RuntimeException: java.lang.IllegalArgumentException at javassist.util.proxy.ProxyFactory.setField(ProxyFactory.java:356) at javassist.util.proxy.ProxyFactory.createClass3(ProxyFactory.java:339) at javassist.util.proxy.ProxyFactory.createClass2(ProxyFactory.java:312) at javassist.util.proxy.ProxyFactory.createClass(ProxyFactory.java:271) View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4251614#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...