How can I config one page per sheet with struts2 and jasperreport

I'm using the struts2 jasperreports plugin and it works well. The problem is that I want to pass exportParameters and I'm not sure how to do that through the plugin. I did it:

@Result(
   name = "XLS", value = "/<package>/Informe.jasper"</package>, 
   type = JasperReportsResult.class, 
   params = {
      "dataSource", "listaResultados", 
      "format", "XLS",      
      "contentDisposition", "attachment;filename=\"Informe.xls\"", 
      "exportParameters", "exportParametersJR" 
   }
)
 
@SuppressWarnings("unchecked")
    public Map<String, String> getExportParametersJR(){
        JRXlsExporter jrXlsExporter = new JRXlsExporter();
        JrXlsExporter.setParameter(
               JRXlsExporterParameter.IS_ONE_PAGE_PER_SHEET,Boolean.TRUE);
        return jrXlsExporter.getParameters();       
    }

I tried, and I got the datasource correctly, but when I add this line

"exportParameters", "exportParametersJR"

it show me this exception:

java.lang.NullPointerException at org.apache.struts2.views.jasperreports.JasperReportsResult.doExecute(JasperReportsResult.java:229) at org.apache.struts2.dispatcher.StrutsResultSupport.execute(StrutsResultSupport.java:178) at com.opensymphony.xwork2.DefaultActionInvocation.executeResult(DefaultActionInvocation.java:348) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:253) at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:50) at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:504) at org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:419) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148) at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869) at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664) at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527) at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80) at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684) at java.lang.Thread.run(Unknown Source)

could anyone help me?