Tomcat Exception (BaseCallHandler marshallException)

I don't understand why I keep having this exception...
Any hint what's wrong?

2010/4/1 PM 07:03:20 org.directwebremoting.dwrp.BaseCallHandler marshallException
Warning : Exception while processing batch
java.lang.IllegalArgumentException: Failed to find parameter: scriptSessionId
      at org.directwebremoting.dwrp.Batch.extractParameter(Batch.java:91)
      at org.directwebremoting.dwrp.Batch.<init>(Batch.java:59)
      at org.directwebremoting.dwrp.CallBatch.<init>(CallBatch.java:46)
      at org.directwebremoting.dwrp.BaseCallHandler.handle(BaseCallHandler.java:72)
      at org.directwebremoting.servlet.UrlProcessor.handle(UrlProcessor.java:120)
      at org.directwebremoting.servlet.DwrServlet.doPost(DwrServlet.java:141)
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
      at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
      at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
      at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
      at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
      at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
      at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
      at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:859)
      at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:574)
      at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1527)
      at java.lang.Thread.run(Thread.java:619)