Exception im Servlet

java.lang.StringIndexOutOfBoundsException: String index out of range: -3
	at java.lang.String.substring(String.java:1762)
	at java.lang.String.substring(String.java:1735)
	at de.webarts.cms.fe.PagePropertyOutput.generateDownloadInfo(PagePropertyOutput.java:1203)
	at de.webarts.cms.fe.GetServlet.doGet(GetServlet.java:348)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:214)
	at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
	at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
	at org.apache.catalina.core.StandardContextValve.invokeInternal(StandardContextValve.java:198)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:152)
	at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
	at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:137)
	at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:118)
	at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:102)
	at de.webarts.valves.SessionValve.invoke(SessionValve.java:76)
	at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:102)
	at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
	at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
	at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:929)
	at org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:162)
	at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:300)
	at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:374)
	at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:743)
	at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:675)
	at org.apache.jk.common.SocketConnection.runIt(ChannelSocket.java:866)
	at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
	at java.lang.Thread.run(Thread.java:595)