IsInRole with custom WebForms authentication for ASP.NET MVC 5

I am trying to implement custom Forms authentication in an ASP.NET MVC 5 app, but I have not done this before and need some help. I am not using the membership provider, but have followed (parts of) http://tech.pro/tutorial/1216/implementing-custom-authentication-for-aspnet. So I am using a CustIdentity and CustPrincipal.

Here is Application_AuthenticateRequest from Global.asax.cs:

protected void Application_AuthenticateRequest(object sender, EventArgs e)
{
    HttpCookie authCookie = Request.Cookies[FormsAuthentication.FormsCookieName];
    if (authCookie != null)
    {
        FormsAuthenticationTicket ticket = FormsAuthentication.Decrypt(authCookie.Value);
        var identity = new CustIdentity(ticket);
        var principal = new CustPrincipal(identity);
        HttpContext.Current.User = principal;
    }
}

It works fine this far, and I can call HttpContext.Current.User.IsInRole("Admin") successfully at the end of the above method.

However, when I try User.IsInRole("Admin") from a view, I get: 'You must call the "WebSecurity.InitializeDatabaseConnection" method before you call any other method of the "WebSecurity" class' Since I do not use the membership provider, WebSecurity.InitializeDatabaseConnection should not apply (I already have my SQL user table, etc. setup).

So I try

var user = User as CustPrincipal;
if (user != null && user.IsInRole("Admin"))
....

But User cannot be cast to CustPrincipal, so always returns null.

What am I missing here? Help would be much appreciated!