Quickbooks AddInvoice XML issue

Question

I've created this XML but the XML validator tells me its invalid. I am told that:

Line: 32
LinePos: 12
Src Text: <RefNumber>INV-0123</RefNumber>
Reason: Element content is invalid according to the DTD/Schema.
Expecting: ExchangeRate, ExternalGUID, LinkToTxnID, SetCredit, InvoiceLineAdd, InvoiceLineGroupAdd, DiscountLineAdd, SalesTaxLineAdd, S....

The XML I am validating is as follows:

<?xml version="1.0" encoding="utf-8"?>
<?qbxml version="13.0"?>
<QBXML>
    <QBXMLMsgsRq onError="stopOnError">
        <InvoiceAddRq>

            <InvoiceAdd defMacro="HEADER">


                <CustomerRef>
                    <FullName>Client Name</FullName>
                </CustomerRef>


                <BillAddress>
                    <Addr1>The building</Addr1>
                    <Addr2>Office 3
                        Chicago</Addr2>
                    <Addr3></Addr3>
                    <Addr4></Addr4>
                    <City></City>
                    <State></State>
                    <PostalCode>60601</PostalCode>
                    <Country>United States</Country>
                </BillAddress>


                <PONumber>PO-123</PONumber>
                <FOB>Bob Smith</FOB>
                <RefNumber>INV-0123</RefNumber>

                <InvoiceLineAdd defMacro="costingID422614">
                    <ItemRef>
                        <FullName>INTERIM</FullName>
                    </ItemRef>
                    <Desc>Interim</Desc>
                    <Quantity>1</Quantity>
                    <Rate>10000</Rate>
                    <Amount>10000</Amount>
                </InvoiceLineAdd>


            </InvoiceAdd>
        </InvoiceAddRq>
    </QBXMLMsgsRq>
</QBXML>

The issue is to do with the RefNumber parameter - I need to specify it but I cannot find where to put it. I've looked at the OSR but the placement still doesn't work.

Thanks for any help you can offer!

Chris