What to say to a potential employer when they ask "What do you NOT like about your current job?"

I had a preliminary phone interview recently. The guy on the phone asked me first, of course, why I'm looking for a new job. I just answered with the standard "looking for new and better opportunity".

After a few more questions, she circled back and said "So, I know you said you're looking for a new opportunity. But is there anything you don't like about your current job now?"

I really had no idea what to say so I made up some crap about the scheduling on the spot. But what is a good professional sounding answer to this? I know I shouldn't bad mouth my current employer at all, so I had no idea how to answer this.