Unable to cast object 'System.Security.Principal.GenericIde'

Question

I am trying to add AD Authentication to a existing Intranet Aplication,

 

The Application uses Form Authentication at the moment. 

 

<authentication mode="Forms">

<forms loginUrl="logon.aspx" name="adAuthName" timeout="60" path="/"> </forms>

</authentication>

 

My Web Application works Correctly but when i go to Login.aspx page this error comes on: 

 System.InvalidCastException: 'Unable to cast object of type 'System.Security.Principal.GenericIdentity' to type 'System.Web.Security.FormsIdentity'.'

 

 

 Error Details :

System.InvalidCastException occurred

HResult=0x80004002

Message=Unable to cast object of type 'System.Security.Principal.GenericIdentity' to type 'System.Web.Security.FormsIdentity'.

Source=App_Web_dusai4r1

StackTrace:

at Account_Login.Page_Load(Object sender, EventArgs e) in C:\Users\d_rody\Desktop\BlueStone.1old\WebSites\BlueStone\Account\Login.aspx.cs:line 16

The code i am using :

 

FormsIdentity id = (FormsIdentity)User.Identity;

FormsAuthenticationTicket ticket = id.Ticket;

Response.Write("<p/>TicketName: " + ticket.Name);

Response.Write("<br/>Cookie Path: " + ticket.CookiePath);

Response.Write("<br/>Ticket Expiration: " +

ticket.Expiration.ToString());

Response.Write("<br/>Expired: " + ticket.Expired.ToString());

Response.Write("<br/>Persistent: " + ticket.IsPersistent.ToString());

Response.Write("<br/>IssueDate: " + ticket.IssueDate.ToString());

Response.Write("<br/>UserData: " + ticket.UserData);

Response.Write("<br/>Version: " + ticket.Version.ToString());

Also I am not a .Net expert, so any help will be greatly appreciated.