Never answer any interview question with "I don't have a reply". There is always an answer. The interviewer isn't looking for the answer, just how you reply. Giving an answer shows you're thorough and have thought through the range of possibilities. For this question, be prepared with about 5 more obscure traits that most interviewers fail to ask, pick one to answer and try to make it tie into the job. Make sure they are positive in every way. Most people interpret this question as seeking something negative, it's not defined that way.