NullPointerException in Servlets.browser(Servlets.java:533), the "userAgent" could be null ?

java.lang.NullPointerException
at org.zkoss.web.servlet.Servlets.browser(Servlets.java:533)
at org.zkoss.web.servlet.Servlets.isBrowser(Servlets.java:503)
at org.zkoss.web.servlet.Servlets.isBrowser(Servlets.java:484)
at org.zkoss.zk.ui.http.WpdExtendlet$WpdContent.toByteArray(WpdExtendlet.java:670)
at org.zkoss.zk.ui.http.WpdExtendlet$WpdContent.access$400(WpdExtendlet.java:631)
at org.zkoss.zk.ui.http.WpdExtendlet.retrieve(WpdExtendlet.java:147)
at org.zkoss.zk.ui.http.WpdExtendlet.service(WpdExtendlet.java:92)
at org.zkoss.web.util.resource.ClassWebResource.web0(ClassWebResource.java:583)
at org.zkoss.web.util.resource.ClassWebResource.web(ClassWebResource.java:569)
at org.zkoss.web.util.resource.ClassWebResource.service(ClassWebResource.java:260)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:417)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at com.researchforgood.delta.Delta.doFilter(Delta.java:183)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.RemoteIpValve.invoke(RemoteIpValve.java:681)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:964)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:304)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:636)