Server Error in '/' Application.

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[NullReferenceException: Object reference not set to an instance of an object.]
   Befroush.<>c__DisplayClass2_0.<Session_Start>b__0(IPList x) in D:\MyProgram\Befroush\Feyzi\Trunk\Trunk\Befroush\Global.asax.cs:173
   System.Linq.WhereListIterator`1.MoveNext() +77
   System.Linq.Enumerable.FirstOrDefault(IEnumerable`1 source) +168
   Befroush.MvcApplication.Session_Start(Object sender, EventArgs e) in D:\MyProgram\Befroush\Feyzi\Trunk\Trunk\Befroush\Global.asax.cs:173
   System.Web.SessionState.SessionStateModule.RaiseOnStart(EventArgs e) +10072608
   System.Web.SessionState.SessionStateModule.CompleteAcquireState() +165
   System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData) +1126
   System.Web.AsyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +261
   System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +50
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +163