OnSelectedIndexChanged事件不会为下拉列表的第一个元素触发

我有一个DropDownList, OnSelectedIndexChanged事件的方法改变了一个标签的内容。除了DropDownList的第一个元素外，它工作得很好。什么好主意吗?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using Newsletter.DAL;
using Newsletter.Services;
public partial class Senders : System.Web.UI.Page
{
    private SenderService _client;
    private List<Sender> _allSenders;
    private DropDownList _dropDownListSenders;
    private TextBox _emailBox;
    private string _selectedSender;
    protected void Page_Load(object sender, EventArgs e)
    {
        _client = new SenderService();
        _allSenders = _client.GetAllSenders();
        _dropDownListSenders = (DropDownList)loginViewMain.FindControl("DropDownListSenders");
        _emailBox = (TextBox)loginViewMain.FindControl("textBoxEmail");
    }
 protected void DropDownListSenders_SelectedIndexChanged(object sender, EventArgs e)
    {
        _selectedSender = _dropDownListSenders.SelectedValue;
        _emailBox.Text = _selectedSender;
    }
}