[gesman]

Tell "it's confidential according to the contract I've signed with my previous employer".

As a compromise - tell: "it's in the ballpark between A and B", where A was your actual salary and your desired salary is in between A and B.

[marioluigi]

I would rather not give a range. Anytime you give a range for a quote (even though this is not a quote, it kind of is), the person on the other side will go with the lower value, rendering your higher value meaningless.