Struts2 + JSON : ERRO

Estou a usar struts2(2.1.8.1) + json (jsonplugin-0.30.jar)

Esta me a dar o seguinte erro:

[#|2010-06-09T10:01:08.329+0100|WARNING|glassfishv3.0|javax.enterprise.system.container.web.com.sun.enterprise.web|_ThreadID=33;_ThreadName=Thread-1;|StandardWrapperValve[default]: PWC1406: Servlet.service() for servlet default threw exception
java.lang.NoSuchMethodError: com.opensymphony.xwork2.ActionContext.get(Ljava/lang/Object;)Ljava/lang/Object;
at com.googlecode.jsonplugin.JSONResult.execute(JSONResult.java:153)

Algum sabe alguma ideia o que possa ser?