That answer will be negative... I'm guessing that you are checking the answer against a textbook answer? 

The thing to realize about textbook answers is that they are generated independently of publication. Typically, graduate students carefully work out the answers to problems that are supposed to be published. From time-to-time problems are changed, such as when a textbook moves from one revision or another. This means there are multiple sources of potential error:

1. the graduate student might have had a bad day and missed a few or might have been provided with an incorrect set of questions

2. the publisher might have made a typo in either the problem or the solution

3. the problem might be changed without updating the solution

You will find many instances where the answer in the back of the book proves to be incorrect. The further along in your studies you get, the more frequent this becomes because the problems become more difficult. It is possible that a bug in the software might leave you with a wrong answer [not very likely], but you will know that you are approaching the answer with the correct problem every time because you are entering it into the program yourself!