Insert a new record (Price Book)

I am trying to insert a new Price Book record from my api.

This is my XML:

<PriceBooks>
    <row no="1">
        <FL val="SMOWNERID">2505249000000119011</FL>
        <FL val="Price Book Owner" />
        <FL val="Price Book Name">My book</FL>
        <FL val="Active" />
        <FL val="Created By" />
        <FL val="Pricing Model">Differential</FL>
        <FL val="Modified By" />
        <FL val="Description" />
        <FL val="Pricing Details" />
    </row>
</PriceBooks>

I can insert successfully if I delete the field "Pricing Model". I have tried change the text for "Flat" or "-None-", but it doesn't work.