Get Object reference when using context rewrite path

Hello

I get this error when i have this in my codebehind: if(Page.User.Identity.IsAuthenticated)

      System.NullReferenceException: Object reference not set to an instance of an object.

This error show when i use context.RewritePath method
http://localhost/page.apsx?id=22 --> http://localhost/hello-world/

Page.User.Identity.IsAuthenticated return True when i go to page.aspx?id=22 but i get

      System.NullReferenceException: Object reference not set to an instance of an object.

error when i go to /hello-world/ url.
...