error with JSP and custom tag

I am running Tomcat 4.1.13 with Java JDK 1.6.  I am trying to add displaytag custom tags and I get the following error:

java.lang.NoClassDefFoundError: org/apache/taglibs/standard/lang/support/ExpressionEvaluatorManager
      at org.displaytag.tags.el.ExpressionEvaluator.eval(ExpressionEvaluator.java:63)
      at org.displaytag.tags.el.ExpressionEvaluator.evalString(ExpressionEvaluator.java:80)
      at org.displaytag.tags.el.ELTableTag.evaluateExpressions(ELTableTag.java:362)
      at org.displaytag.tags.el.ELTableTag.doStartTag(ELTableTag.java:348)
      at org.apache.jsp.workflowEvent_jsp._jspService(workflowEvent_jsp.java:717)
      at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:92)
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:809)
      at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:162)
      at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:240)
      at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:187)
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:809)
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:200)
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:146)
      at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:209)
      at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
      at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
      at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
      at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:144)
      at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
      at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
      at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
      at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2358)
      at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:133)
      at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
      at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:118)
      at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
      at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:116)
      at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
      at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
      at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
      at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:127)
      at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
      at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
      at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
      at org.apache.coyote.tomcat4.CoyoteAdapter.service(CoyoteAdapter.java:152)
      at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:799)
      at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:705)
      at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:577)
      at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:683)
      at java.lang.Thread.run(Thread.java:619)

Can you tell me why I get this error?