<ww:form>标签问题

<ww:form namespace="'wwyanzhen'" action="'register.action'" method="'POST'" >
如果把action这个属性加上就会出现以下错误：
HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:370)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
root cause java.lang.NullPointerException
com.opensymphony.webwork.views.jsp.ui.FormTag.evaluateExtraParams(FormTag.java:122)
com.opensymphony.webwork.views.jsp.ui.AbstractUITag.evaluateParams(AbstractUITag.java:408)
com.opensymphony.webwork.views.jsp.ui.AbstractUITag.doEndTag(AbstractUITag.java:204)
org.apache.jsp.registerSupport_jsp._jspx_meth_ww_form_0(org.apache.jsp.registerSupport_jsp:190)
org.apache.jsp.registerSupport_jsp._jspService(org.apache.jsp.registerSupport_jsp:107)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:322)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
note The full stack trace of the root cause is available in the Apache Tomcat/5.5.9 logs.
--------------------------------------------------------------------------------Apache Tomcat/5.5.9
这是怎么回事呀。
把action这个属性删除后就一切正常了。