Error :"Unable to cast object of type 'X' to type 'X' "

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
        If Not IsPostBack Then
            If Request.IsAuthenticated Then
                Dim ident As FormsIdentity = CType(User.Identity, FormsIdentity)
                If ident IsNot Nothing Then
                    Dim ticket As FormsAuthenticationTicket = ident.Ticket
                    Dim userDataString As String = ticket.UserData
                    Dim userDataPieces() As String = userDataString.Split("|".ToCharArray())
                    Dim username As String = userDataPieces(1)
                    Dim rolename As String = userDataPieces(2)
                    If rolename <> "Graduate" Then
                        Response.Redirect("~/login.aspx", False)
                    End If
                End If
            Else
                Response.Redirect("~/login.aspx", False)
            End If
            lblerror.Visible = False
            Dim ident1 As FormsIdentity = CType(User.Identity, FormsIdentity)
            Dim ticket1 As FormsAuthenticationTicket = ident1.Ticket
            Dim userDataString1 As String = ticket1.UserData
            Dim userDataPieces1() As String = userDataString1.Split("|".ToCharArray())
            Dim userid As Integer = userDataPieces1(0)
            Dim ds As DataSet

            sqlquery("select offer.offerid as 'Offer ID', companyname + '(' + convert(varchar(50), company.companyid) + ')' as 'Company', request.requestid as 'Request ID', fname + ' ' + lname as 'Student', days as 'Days', minsalary as 'Minimum Salary', maxsalary as 'Maximum Salary' from company, offer, request, requestdetail, users where offer.offerid = requestdetail.offerid and offer.companyid = company.companyid and offer.userid = '" & userid & "' and request.userid = users.userid and request.requestid = requestdetail.requestid;")
            If ds.Tables(0).Rows.Count <> 0 Then
                gvrequestgrad.DataSource = ds.Tables(0)
                gvrequestgrad.DataBind()
                gvrequestgrad.SelectedIndex = -1
            End If 

-Unable to cast object of type 'System.Security.Principal.WindowsIdentity' to type 'System.Web.Security.FormsIdentity'.

- Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

-Exception Details: System.InvalidCastException: Unable to cast object of type 'System.Security.Principal.WindowsIdentity' to type 'System.Web.Security.FormsIdentity'.