Patrick's comment is poignant:
"If you give a number in response to this question, that’s going to immediately cap your negotiator range. If you say “My current salary is 90,” then they know that anything above 90 is an improvement to you. Even if they would have been willing to pay, let’s say 110 to the right candidate for this position, they’re probably going to offer you 95 and then walk it up to 98, 100."
The employer is incentivized to pay you as little as they can. We know that many employers pay different people doing the same job different salaries. If you can get them to give their number first, it seems (to me) you're in a much stronger position.