Making the correct answer blank for a test question

I am working on a test that requires participants to list multiple payments. To be able to measure whether or not they are reading supporting documents correctly, I want to list more payment fields than are needed. The purpose for this is to give the participant the "chance" to list more payments than are required. If they list a payment, when one was not made, the answer would be incorrect. From what I can determine, this would mean that in one of the fields in which no answer (payment) was required, the correct answer would have to be no answer at all. When creating this question though, I am required to place an answer. Does anyone have any advice on how to address this?

Thanks,

Brian