ThreadError: Attempt to unlock a mutex which is not locked

[hungle00]

I copied and ran the sample on the page https://socketry.github.io/async-redis/guides/data-structures/index.
It still shows the result, but displays an error message at the end

 0.01s     warn: Async::Task: Async::Pool::Controller Gardener ...
               | Task may have ended with unhandled exception.
               |   ThreadError: Attempt to unlock a mutex which is not locked

The code using async still works well, so I think the error comes from async-redis gem.