I am getting the error: Error 375 'System.Web.Providers.Entities.User' does not contain a definition for 'Identity'. How do I fix it?

In codebehind, I have 

protected void Page_Load(object sender, EventArgs e)
        {
            if (User.Identity.IsAuthenticated)
            {               
            }
        }

I get the following error with regard to "Identity"

System.Web.Providers.Entities.User' does not contain a definition for 'Identity

I have the exact same "using's" of other forms without a problem, and there is no "resolve" to find one to fix the problem, so I am at a loss as to what is going on.