字段索引问题，帮帮忙啊！

Dim WithEvents adoPrimaryRS As Recordset
Private Sub Form_Load()
Dim cn As ADODB.Connection
Dim f As ADODB.Field
Field f = New ADODB.Field
Dim n As Integer
Set cn = New Connection
cn.CursorLocation = adUseClient
cn.Open "PROVIDER=Microsoft.Jet.OLEDB.3.51;Data Source=D:\VB98\ALB.mdb;"
n = CInt(Text1.Text)
Set f = adoPrimaryRS.Fields(n)
Set adoPrimaryRS = New Recordset
adoPrimaryRS.Open "select avg(len(f)) from ¹¤×ʱí", db, adOpenStatic, adLockOptimistic
Set DataGrid1.DataSource = adoPrimaryRS
DataGrid1.ReBind
End Sub
我想做一个对字段的索引，第5行总是运行不过去，但这是照着MSDN做的，很着急呀
个位高手帮帮忙啊！