Error in file imageStore.swift

I downloaded the code files for the 6th edition of iOS Programming. But when trying to run the code for Chapter 15 (Camera), I got a runtime error. It turns out, the file imageStore.swift has a typo. On line 9, the statement:

let cache = NSCache<String, UIImage>()

should be:

let cache = NSCache<NSString, UIImage>() 

Making the change fixes the error. (The file creation date shown for the file I downloaded is November 1, 2016. )