Why does @ Html.AntiForgeryToken () return null?

I have configured my asp.net mvc application in iis on our server. after starting the application, the exception occurs as follows:

Object reference not set to an instance of an object.
Line 10: {
Line 11:     <div class="form-horizontal">
Line 12:         @Html.AntiForgeryToken()  <-- here

      

        
        
        
      

    

I checked the MachineKey (in the web.config file) of the application in iis, which:

<machineKey decryptionKey="AutoGenerate,IsolateApps" validation="SHA1" validationKey="AutoGenerate,IsolateApps" />

      

        
        
        
      

    

I can't figure out what the problem is!

Update:

also, I checked the session, but it is not null. I got Session.SessionID

in the razor and I got the actual value ( hutqqygxwbvvwaerztvamf1n

).