Error creating a "btw" payment

when i try to "explain a transaction".from my banking section (imported abn amro statements). The problem shows up when i want to explain a BTW payment.. I choose "payment" and manually put the VAT on "0.00". Then i choose "BTW" as the category.

But; when i try to save this payment it gives me a "1 error prohibited this bank account entry from being saved. There were problems with the following fields:Entry Type is invalid".

So it looks like I can't leave the VAT area blank (0.00), Weird thing is that I already have several other bookings exactly like this one but that didn't give me any problems.

Could you guys please give me some advice at this one? Quite frustrating.