Create a new questionnaire with, at least, one mandatory rate question with n rows (n issues to rate) and k columns + the N/A column.
Go to fill your just made questionnaire.
1) Take care to forget to rate, at least, one item.
2) Take care to rate some item(s) selecting the N/A column.
Go to submit.
The questionnaire, as it is supposed to do, will be shown again with a message informing the user about an error in the rate question.
By checking closely the rate question you will see that all the items originally rated with N/A (because of the point 2 stated before) are now shown as rated with the value of the first column.
Attached are two screenshot explaining the situation.