Problem creating an XML file form a datatable

I'm missing something that I've spent the last few hours stuck on, hopefully someone can point me in the right direction.

I'm trying to create an xml document to be opened later by excel.
I can do this with dataset.writexml, but I only want one table written to the xml file.

I get an error on this line.
           rootElement.AppendChild(rowElement)
The error is Object reference not set to an instance of an object.
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

I suspect the problem to be with this line.
        Dim rootElement As XmlElement = doc.DocumentElement
I just don't see what i need to do to get this to work.

Code

        Dim dsBrowse As DataSet = CType(Session("dsBrowse"), DataSet)
        Dim strFilename As String
        strFilename = "C:\Customers2.xml"
        Dim dt As New DataTable
        Dim row As DataRow
        Dim rowElement As XmlElement
        dt = dsBrowse.Tables("tblContact")
        Dim doc As XmlDocument = New XmlDocument
        Dim xw As XmlTextWriter = New XmlTextWriter(strFilename, System.Text.Encoding.UTF8)
        Dim rootElement As XmlElement = doc.DocumentElement()
        Dim i As Integer, cnt As Long
        Dim colElement(dt.Columns.Count) As XmlElement
        xw.WriteProcessingInstruction("xml", "version='1.0'")
        xw.WriteProcessingInstruction("mso-application", "progid='Excel.Sheet'")
        For Each row In dt.Rows
            rowElement = doc.CreateElement("Table")
            For i = 0 To dt.Columns.Count - 1
                colElement(i) = doc.CreateElement(dt.Columns(i).Caption)
                colElement(i).InnerText = row(i).ToString()
                rowElement.AppendChild(colElement(i))
            Next i
            rootElement.AppendChild(rowElement)
        Next
        doc.WriteTo(xw)
        xw.Close()