No mapping exists from object type System.Web.UI.WebControls.TextBox to a known managed provider native type.

i''m trying to send en email when the the textbox value is set to valid when the user click save but there is this error don''t know what wrong 
Imports System.Data Imports System.Data.SqlClient Imports System.Configuration Imports System.Net.Mail Imports System.Web.Configuration Imports System.Web.Script.Services Imports System.Web.Services Partial Class Admin_EditAuction Inherits System.Web.UI.Page Dim con As New SqlConnection(ConfigurationManager.ConnectionStrings("VirgoPlaza").ToString()) Dim EndDate As DateTime Dim auctionemail As String Dim temp1 As Integer Private ReadOnly _start As String Public Sub New() _start = WebConfigurationManager.ConnectionStrings("VirgoPlaza").ConnectionString End Sub Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load If Not Page.IsPostBack Then BindItems() ...