Do not say you have none - this will lead to further problems. You have two options - use a professed weakness such as a lack of experience (not ability) on your part in an area that is not vital for the job. The second option is to describe a personal or professional weakness that could also be considered to be a strength and the steps you have taken to combat it. An example would be: "I know my team think I'm too demanding at times - I tend to drive them pretty hard but I'm getting much better at using the carrot and not the stick".