Tomcat ApplicationContext getInitParameter throws NullPointerException

When I try to start my Web-Application from within eclipse, I get the following exception:

 SCHWERWIEGEND: Exception sending context initialized event to listener instance of class org.springframework.web.util.Log4jConfigListener
java.lang.NullPointerException
    at java.util.concurrent.ConcurrentHashMap.put(Unknown Source)
    at org.apache.catalina.core.ApplicationContext.mergeParameters(ApplicationContext.java:883)
    at org.apache.catalina.core.ApplicationContext.getInitParameter(ApplicationContext.java:255)
    at org.apache.catalina.core.ApplicationContextFacade.getInitParameter(ApplicationContextFacade.java:299)
    at org.springframework.web.util.Log4jWebConfigurer.exposeWebAppRoot(Log4jWebConfigurer.java:185)
    at org.springframework.web.util.Log4jWebConfigurer.initLogging(Log4jWebConfigurer.java:116)
    at org.springframework.web.util.Log4jConfigListener.contextInit开发者_如何学Pythonialized(Log4jConfigListener.java:45)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:3934)
    at org.apache.catalina.core.StandardContext.start(StandardContext.java:4429)
    at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1045)

When I start it with jetty, there is no Problem. Since it took me some hours to find the problem, I'm posting it here, in order to find the answer sooner next time.

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The problem was, that one of the parameters in the context.xml did not have a "value" attribute.