A Requested Title the Library Doesn't Own

A patron came in today and asked if we had a book. I had to say that we didn't but we have a similar title. The patron said they didn't want the new book by that author, they wanted the original book that was published in the 1950s called:
To Kill a Hummingbird.

The patron asked since we don't have it, could we order it. I explained no, and why. Even though it should exist, that book doesn't.