i am having difficulty updating a quote's product detail ... i am getting the very-descriptive 4500 error code.

here is the XML i am passing in:

<Quotes>
    <row no="1">
        <fieldlabel value="Sub Total">123.45</fieldlabel>
        <fieldlabel value="Product Details">
            <product no="1">
                <fieldlabel value="Product Id">111111000000000001</fieldlabel>
                <fieldlabel value="List Price">123.45</fieldlabel>
                <fieldlabel value="Unit Price">123.45</fieldlabel>
                <fieldlabel value="Total">123.45</fieldlabel>
                <fieldlabel value="Total After Discount">123.45</fieldlabel>
                <fieldlabel value="Net Total">123.45</fieldlabel>
            </product>
        </fieldlabel>
        <fieldlabel value="Modified Time">2009-12-18 15:45:31</fieldlabel>
    </row>
</Quotes>

(note: productId is an actual productId in my ZohoCRM, so that's not the issue)

anyone else been able to successfully do this?

cheers,
-e