Server Error in '/' Application.

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
Source Error:
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace:
[NullReferenceException: Object reference not set to an instance of an object.]
GuoDao.Global.Session_Start(Object sender, EventArgs e) in e:\tfs\website\GuoDao\GuoDao\Global.asax.cs:25
System.Web.SessionState.SessionStateModule.RaiseOnStart(EventArgs e) +9107283
System.Web.SessionState.SessionStateModule.CompleteAcquireState() +148
System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData) +561
System.Web.AsyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +96
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184
Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.1031