我用VB语言编写了一个程序，在前台添加了一个dropdownlist控件（Dlst用户名），里面的值在Page_load时从数据库提取赋值，然后点击“登陆”按钮后，把dropdownlist中选定的值作为“用户名”条件输入，用户输入密码，然后判断是否通过验证。但是发现，我这个程序运行后点击“登陆”，VS就报错：“reader不存在该行/列”（红字部分，我用来检测哪里出问题的代码）。如果去除where后面的“ 用户名='" & Me.Dlst用户名.Text & "' ”这个条件，程序就可以正常取得reader的值了，请问这是怎么回事？
Imports System.Data
Imports System.

Partial Public Class login

    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Me.Dlst用户名.Items.Clear()
        '初始化下拉菜单

        Initlst()

    End Sub

    Public Sub Initlst()
        Dim engin As New MyDbEngine
        Dim cmdText As String
        cmdText = "select 用户名 from 用户信息表"
        engin.OpenConnect()
        Dim reader As Common.DbDataReader = engin.ExecuteReader(cmdText)
        reader.Read()
        If IsNothing(reader) Or reader.HasRows = False Then
            Response.Write("没有用户信息！")
            reader.Close()
            engin.CloseConnect()
            Exit Sub
        End If
        reader.Close()
        reader = engin.ExecuteReader(cmdText)
        While reader.Read
            Me.Dlst用户名.Items.Add(reader("用户名").ToString)
        End While
        reader.Close()
        engin.CloseConnect()
    End Sub

    Protected Sub btn登陆_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btn登陆.Click
        Dim engin As New MyDbEngine
        Dim cmdText As String
            cmdText = "select * from 用户信息表 where 密码='" & Me.txt密码.Text.Trim & "' and 用户名='" & Me.Dlst用户名.Text & "'"
            engin.OpenConnect()
            Dim reader As Common.DbDataReader = engin.ExecuteReader(cmdText)
            reader.Read()
            Response.Write(reader("用户名"))
            If IsNothing(reader) Or reader.HasRows = False Then
                Response.Write("用户名或密码错误！")
                reader.Close()
                engin.CloseConnect()
                Exit Sub

            End If
            Response.Write("验证通过")
            reader.Close()
        engin.CloseConnect()
    End Sub
End Class