It has become quite common for employers to ask prospect employees this question in an interview. They sometimes claim this is to see whether they can fit you within their budget, but in my opinion that is an excuse:
The really relevant information for them would be what salary you expect from them, not what your salary was with the last employer. Your current/former salary is only relevant for someone who wants to lowball you; in order to gauge whether you fit into their budget, they only need to know what salary you expect from them.
This is why I never disclose my current/former salary in an interview; I always deflect that question by stating my expected salary instead. So far, I only had one situation where a prospect employer insisted on getting information about my current salary (which was a red flag for me) so I revoked my application for the job.
Last but not least, I have often seen deployment contract paragraphs specifically forbidding the employee to reveal their salary with any third party (except government agencies); so I'd say it's definitely not unprofessional of you to refuse to reveal that information.