Auto GenID LINQ to SQL VB.NET ครับ ไม่ทำงานครับ

 Dim FirstID As String = ""
            Dim emp_add As New employee()
            Dim emp_id = From id In db.employees _
                                            Order By id.employee_id Descending Take (1)
            Dim LastEmpID As String = ""

            If emp_id.Count() = 1 Then
                FirstID = emp_id.SingleOrDefault().employee_id.Left(4)
                LastEmpID = String.Format(FirstID, "0000")
                FirstID = +1
            End If
            emp_add.employee_id = LastEmpID
            emp_add.employee_fullname = txt_fullname.Text.Trim