The workplace is not a courtroom, so you can't plead the fifth when you're asked about your negative qualities. We all have them, after all, and your interviewer knows this as well!
The key to answering this question is to turn a negative into a positive. For instance, you may tell the interviewer that you're often too hard on yourself. Rather than simply being viewed as a negative quality, the employer may consider that attribute to be a sign of conscientiousness.