It [has been reported](https://github.com/evgenyneu/keychain-swift/issues/15) that the library sometimes returns `nil` instead of the stored Keychain value. The issue seems to be random and hard to reproduce. It may be connected with [the Keychain issue](https://forums.developer.apple.com/thread/4743) reported on Apple developer forums.
Evgenii Neumerzhitckii