如何处理SQL查询返回空值的问题，求助！

WinFrm过程代码：
    public myItem as String    '用于返回值的公共变量
    Public Sub DatSearch(ByVal TabName As String, ByVal Field As String, ByVal Guide As String)
        CmdText = "Select " & Field & " From " & TabName & " Where " & Guide
        If Conn.State = 1 Then Conn.Close() ‘若Conn连接，则关闭它
        Dim Cmd As SqlClient.SqlCommand = New SqlClient.SqlCommand(CmdText, Conn)’新建数据连接
        SQLAdp.SelectCommand = New SqlClient.SqlCommand(CmdText, Conn)‘新建数据查询命令
        Try
            Conn.Open()                                                           '连接数据库
            SQLAdp.Fill(DataSet, "SearchTab")                        '填充数据集缓存
            Dim myResp As Object = Cmd.ExecuteScalar         '判断是否存在记录

       if myResp Is System.DBNull Then
                     myItem =Nothing                                   '如果myResp为空
            Else
                    myItem =DataSet.Tables("SearchTab").Rows(0).Item(0)
            End if

            DataSet.Tables("SearchTab").Clear()
            DataSet.Tables.Remove("SearchTab")
            Conn.Close()
        Catch ex As Exception
            MsgBox(ex.Message)
            Exit Try
        End Try
    End Sub

上述代码在执行时，当查询返回空记录时，会出错：“在0处没有任何行！”，问题在哪里？
怎样才能处理出现空值或空记录的情况，谢谢高手指点！