Write a structure to an xml file: "XML document must have a top level element" error

Hi, i try to write an xml file. When trying to read the file the following error is shown in the browser:-

XML document must have a top level element"

The coding i'm using is

                Dim xmlFilename As String = "myXmlDocument.xml"
            ' Create FileStream    
            Dim fsWriteXml As New System.IO.FileStream(xmlFilename, System.IO.FileMode.Create)
            ' Create an XmlTextWriter to write the file.
            Dim xmlWriter As New System.Xml.XmlTextWriter(fsWriteXml, System.Text.Encoding.Unicode)
            ' Use WriteXml to write the document.
            xmlWriter.WriteProcessingInstruction("xml", "version='1.0' encoding='ISO-8859-1'") ' eerste regel geschreven
            ' write the root
            xmlWriter.WriteStartElement("aaa")
            xmlWriter.WriteElementString("date", Nothing, "2006-11-03 12:17:09")
            For Each Datarow In TableSelected.Rows
                xmlWriter.WriteStartElement("client")
                xmlWriter.WriteElementString("id", Nothing, "2006-11-03 12:17:09")
                For Each Datarow2 In Table2Selected.Rows
                    Dim internnr As Integer =Datarow2.Item(1)
                    xmlWriter.WriteElementString("clientnr", Nothing, internnr)
                    xmlWriter.WriteStartElement("profiel")
                    For Each Datarow2 In Table3tSelected.Rows
                        xmlWriter.WriteStartElement("profiel_line")
                        xmlWriter.WriteElementString("id", Nothing, "0000011218")
 .                      ..... more elements .....
                        xmlWriter.WriteEndElement() ' einde profiel_line
                    Next
                    xmlWriter.WriteEndElement() ' end profiel
                    xmlWriter.WriteEndElement() ' end client ...

                Next
                xmlWriter.WriteEndElement() ' end farmdossier
                fsWriteXml.Flush()

                fsWriteXml.Close()

What am i doing wrong.  BTW: i'm not an XML expert