NullReferenceException in AppenderSkeleton.get_Name()

I had an ASP.NET 1.1 application crash recently. There was an
unhandled exception in AppenderSkeleton.get_Name(). I don't think this
is the cause of the application failure but I am hoping it might help
me determine the root cause of the problem.

What could cause this exception in this method?

System.NullReferenceException: Object reference not set to an instance
of an object.
  at log4net.Appender.AppenderSkeleton.get_Name()
  at log4net.Util.AppenderAttachedImpl.AppendLoopOnAppenders(LoggingEvent
loggingEvent)
  at log4net.Repository.Hierarchy.Logger.CallAppenders(LoggingEvent
loggingEvent)
  at log4net.Repository.Hierarchy.Logger.ForcedLog(Type
callerStackBoundaryDeclaringType, Level level, Object message,
Exception exception)
  at log4net.Repository.Hierarchy.Logger.Log(Type
callerStackBoundaryDeclaringType, Level level, Object message,
Exception exception)
  at log4net.Core.LogImpl.Error(Object message, Exception exception)

Thanks
Kevin