实时异常91，“对象变量或with变量未设置”是咋回事

实时错误91，“对象变量或with变量未设置”是怎么回事？
模块：
Public cn As ADODB.Connection 
Public rs As ADODB.Recordset  
Sub main()
  Set cn = New ADODB.Connection  
  cn.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" & App.Path & "\personinfo.mdb;Persist Security Info=False"  
  cn.Open '打开连接
   
  Set rs = New ADODB.Recordset
  frmdl.Show
End Sub

登录窗体
Private Sub Command1_Click()

  rs.Open "select * from user_tbl where u_name='" & Combo2.Text & "'", cn, adOpenStatic, adLockReadOnly
  If rs.RecordCount > 0 Then

  If Text1(1).Text = rs.Fields("u_password").Value Then
  Unload Me
  frmgr.Show
  MDIForm1.Show
  Else
  MsgBox "用户名或密码错误！", vbInformation, "提示"
  End If
  End If
   
End Sub
请前辈指点！

[解决办法]
错误出在那一行？按F8单步调试一下
[解决办法]
http://download.csdn.net/source/1498324
[解决办法]

VB code

'模块 codeOption Explicit    Public cn As New ADODB.Connection    Public rs As New ADODB.RecordsetSub main()  cn.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" & App.Path & "\personinfo.mdb;Persist Security Info=False"  cn.Open '打开连接  frmdl.ShowEnd Sub'登录窗体Option ExplicitPrivate Sub Command1_Click()    sql = "select * from user_tbl where u_name='" & Combo2.Text & "'"    rs.CursorLocation = adUseClient    rs.Open sql, cn, adOpenDynamic, adLockOptimistic    If rs.RecordCount > 0 Then        If Text1(1).Text = rs.Fields("u_password").Value Then            Unload Me            frmgr.Show            MDIForm1.Show        Else            MsgBox "用户名或密码错误！", vbInformation, "提示"        End If    End IfEnd Sub
[解决办法]
VB code
'模块 codeOption Explicit    Public cn As ADODB.Connection '定义数据库的连接存放数据和代码    Public rs As ADODB.Recordset    Public sql As StringSub main()    Set cn = New ADODB.Connection    cn.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" & App.Path & "\personinfo.mdb;Persist Security Info=False"    cn.Open '打开连接    Set rs = New ADODB.Recordset    frmdl.ShowEnd Sub