"I didn't finish it.""So was I."<=Why is the answer right?

Dear all,

I've just tried the testing function in the website and there is a question of “I didn’t finish it.” The four choices to this question are “So was I”, “So did I”, “Neither was I” and “Neither did I” and the answer to it is “So was I.”

Why is it so? I remember the grammar books suggest as a negative sentence you have to use “neither” to show you are in the same situation and “I didn’t finish it” is a negative sentence with an auxiliary verb “did”. Therefore, I’ve thought the answer should be “Neither did I.” Can someone explain the answer to me? Thanks.

The answer should be "Neither did I." It could be a computer error.