What does this error means?

Im doin jsp using Tomcat4.1 and j2sdk1.4.1_02.. When i try to add a record, it will display this error:

org.apache.jasper.JasperException: Adding text to an XML document must not be null

at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:254)

at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:295)

at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:256)

at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:643)

at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:480)

at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:995)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)

at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:643)

at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:480)

at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:995)

at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2415)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:180)

at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:643)

at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:171)

at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:641)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:172)

at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:641)

at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:480)

at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:995)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:174)

at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:643)

at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:480)

at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:995)

at org.apache.coyote.tomcat4.CoyoteAdapter.service(CoyoteAdapter.java:223)

at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:594)

at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:392)

at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:565)

at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:619)

at java.lang.Thread.run(Unknown Source)

root cause

java.lang.IllegalArgumentException: Adding text to an XML document must not be null

at org.dom4j.DocumentFactory.createText(DocumentFactory.java:136)

at org.dom4j.tree.AbstractElement.addText(AbstractElement.java:740)

at org.apache.jsp.SaveStaff_jsp._jspService(SaveStaff_jsp.java:83)

at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:137)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)

at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:210)