Modify SalesOrder, DI Server, Invalid valid value '' in Enum 'BoYesNoEnum'

Question

I'm trying to modify salesorders through the DI Server.
I've added ordres through the DI Server with no errors, but modifying them raises an exception.
Anyone who know what the "Invalid valid value '' in Enum 'BoYesNoEnum'" is? Here is some simple XML:
<?xml version="1.0" encoding="UTF-16"?>
<env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">
<env:Header>
<SessionID>1C38F764-D521-19C0-9401-3C083AE414DC</SessionID>
</env:Header>
<env:Body>
<dis:UpdateObject xmlns:dis="http://www.sap.com/SBO/DIS">
<BOM>
<BO>
<AdmInfo>
<Object>oOrders</Object>
</AdmInfo>
<QueryParams>
<DocEntry>66</DocEntry>
</QueryParams>
<Documents>
<row>
<Comments>Test</Comments>
</row>
</Documents>
</BO>
</BOM>
</dis:UpdateObject>
</env:Body>
</env:Envelope>
This should have worked, in my opinion.
However, I just get "Invalid valid value '' in Enum 'BoYesNoEnum'".

Answer 1

Ah, finally solved it, after ALOT of debuggin!
After adding the following line:
<ApplyTaxOnFirstInstallment>tNO</ApplyTaxOnFirstInstallment>
it finally worked. Don't know what impact that line has though, but hopefully nothing serious. Strange that this thing doesn't get set by default..