Treeview Duplicates

I'm new to VB.NET (VB6 programmer) and from the various threads i've searched, I have put together the code below to catch duplicates in order to populate a treeview. I'm after the following result:

Audi
- A4
- A5
BMW
- 1 Series
- 3 Series

My data is simply two columns: Audi, A4, Audi, A5, BMW, 1 Series, etc...

Code below:
'-----------------------------------------------------------------
Dim parentrow As DataRow
Dim ParentTable As DataTable
ParentTable = ds.Tables("Makes")
Dim parentnode As TreeNode

Dim lastName As String
lastName = ""
For Each parentrow In ParentTable.Rows
For Each row As DataRow In ParentTable.Rows
If CStr(row("Manufacturer")) <> lastName Then
parentnode = New TreeNode(parentrow.Item(0))
Else
parentnode.Nodes.Add(parentrow.Item(1))
End If
lastName = CStr(row("Manufacturer"))
Next
Next
'----------------------------------------------------------------

Unfortunately the parentnode in the Else part of the code is showing "variable used before assigned". Any help with the above would be appreciated.

Thanks in advance.

[RESOLVED]

Code:

---

Dim lastName As String
        Dim lastModel As String
        lastModel = ""
        lastName = ""
        For Each parentrow In ParentTable.Rows
            If CStr(parentrow("Manufacturer")) <> lastName Then
                parentnode = New TreeNode(parentrow.Item(0))
                TreeView1.Nodes.Add(parentnode)
            Else
                parentnode.Nodes.Add(parentrow.Item(1))
            End If
            lastName = CStr(parentrow("Manufacturer"))
        Next

---