对象变量或with块变量未定义

Public Function BtnDisplay()

    Dim btn As Label, H As Integer
    Dim SQL As String
    Dim db As Database
    Dim rec As DAO.Recordset
    Dim bTop As Integer

    SQL = "SELECT Menu.Number, Menu.Name, Left([Number],1) AS btn FROM Menu WHERE (((Menu.Name) Is Not Null) AND ((Left([Number],1))=" & BT & "));"
   
    Set rec = db.OpenRecordset(SQL, dbOpenDynaset)
   
    Do While Not rec.EOF
        H = H + 1
        Set btn = Me.Controls("AB" & Right(rec(0), 1))
   
        If H = 1 Then
            bTop = 2800
        Else
            bTop = bTop + 500
        End If
       
        btn.Visible = True
        btn.Top = bTop
        btn.Caption = rec(1)
        rec.MoveNext
    Loop
   
    Box1.Visible = True
    Box1.Height = BoxH(H)
    rec.Close
    Set rec = Nothing
    Set db = Nothing
   

End Function

以上是程序，运行时出现 运行时错误‘91’，对象变量或with块变量未设置， 光标停在Set rec = db.OpenRecordset(SQL, dbOpenDynaset)

求高手解答。。。