Problem comparing xml data vb6

Run-time error  91:

Object variable or with lock variable not set

If objDoc.transformNode(objDoc) <> objDocCopy.transformNode(objDocCopy) Then

Private Sub Form_Load()


    'Set up the listview
    ListView1.View = lvwReport
    ListView1.ColumnHeaders.Add , , "Artist"
    ListView1.ColumnHeaders.Add , , "Name"
    ListView1.ColumnHeaders.Add , , "Image"
    ListView1.ColumnHeaders.Add , , "Rating"
    ListView1.ColumnHeaders.Add , , "Song ID"
    ListView1.ColumnHeaders.Add , , "Total Votes"
    ListView1.ColumnHeaders.Add , , "Page"
    ListView1.ColumnHeaders.Add , , "Referrer"
    ListView1.ColumnHeaders.Add , , "pageWindowName"
    XMLCompair
   
    'Timer1.Interval = 60000 ' <-- one minute
    Timer1.Interval = 7000 ' <-- 10 seconds
    Timer1.Enabled = True
    
   
End Sub


Private Sub XMLCompair()
  Dim objDoc As MSXML2.DOMDocument
Dim objDocCopy As MSXML2.DOMDocument

  'load the xml document
  Set objDoc = New MSXML2.DOMDocument
  objDoc.async = False
  objDoc.Load "http://localhost/data.php"



If objDoc.transformNode(objDoc) <> objDocCopy.transformNode(objDocCopy) Then
     Set objDocCopy = objDoc
     PopulateListview
End If
End Sub

Private Sub PopulateListview()
Dim objDoc As MSXML2.DOMDocument
Dim objNodelist As IXMLDOMNodeList
Dim objNode As IXMLDOMNode
Dim lvwItem As ListItem

    'load the xml document
    Set objDoc = New MSXML2.DOMDocument
    objDoc.async = False
    objDoc.Load "http://localhost/data.php"
    
    'add all the song nodes into a  nodelist
    Set objNodelist = objDoc.selectNodes("//song")
    
    'Clear the listview
    ListView1.ListItems.Clear

    'Loop through each song node and add to the list view
    For Each objNode In objNodelist
        Set lvwItem = ListView1.ListItems.Add(, , objNode.selectSingleNode("artist").Text)
        lvwItem.SubItems(1) = objNode.selectSingleNode("name").Text
        lvwItem.SubItems(2) = objNode.selectSingleNode("image").Text
        lvwItem.SubItems(3) = objNode.selectSingleNode("rating").Text
        lvwItem.SubItems(4) = objNode.selectSingleNode("songid").Text
        lvwItem.SubItems(5) = objNode.selectSingleNode("totalvotes").Text
        lvwItem.SubItems(6) = objNode.selectSingleNode("page").Text
        lvwItem.SubItems(7) = objNode.selectSingleNode("referrer").Text
        lvwItem.SubItems(8) = objNode.selectSingleNode("pageWindowName").Text
    Next objNode

    Set lvwItem = Nothing
    Set objNodelist = Nothing
   Set objDoc = Nothing
End Sub