nio4r terminates with unsupported backend (ArgumentError) with Puma

I used bundle update to update my gemfile's development support and test gems, as all the production gems are version locked. After the update, nio4r remained at 2.5.2 as it had been locked. However, after the update, nio4r failed with message: nio4r-2.5.2/lib/nio/selector.rb:17:in 'initialize': unsupported backend: (ArgumentError)

The stack trace is as follows:
(Note, this is an effort to upgrade Ruby and Rails, starting by getting testing right & refactoring base.)

Puma starting in single mode...
* Puma version: 6.3.0 (ruby 2.7.2-p137) ("Mugi No Toki Itaru")
*  Min threads: 5
*  Max threads: 5
*  Environment: development
*          PID: 11436
* Listening on http://[::1]:3001
* Listening on http://127.0.0.1:3001
Exiting
Traceback (most recent call last):
        20: from bin/rails:4:in `<main>'
        19: from bin/rails:4:in `require'
        18: from ruby/2.7.0/gems/railties-5.2.5/lib/rails/commands.rb:18:in `<top (required)>'
        17: from ruby/2.7.0/gems/railties-5.2.5/lib/rails/command.rb:46:in `invoke'
        16: from ruby/2.7.0/gems/railties-5.2.5/lib/rails/command/base.rb:69:in `perform'
        15: from ruby/2.7.0/gems/thor-0.20.3/lib/thor.rb:387:in `dispatch'
        14: from ruby/2.7.0/gems/thor-0.20.3/lib/thor/invocation.rb:126:in `invoke_command'
        13: from ruby/2.7.0/gems/thor-0.20.3/lib/thor/command.rb:27:in `run'
        12: from ruby/2.7.0/gems/railties-5.2.5/lib/rails/commands/server/server_command.rb:142:in `perform'
        11: from ruby/2.7.0/gems/railties-5.2.5/lib/rails/commands/server/server_command.rb:142:in `tap'
        10: from ruby/2.7.0/gems/railties-5.2.5/lib/rails/commands/server/server_command.rb:147:in `block in perform'
         9: from ruby/2.7.0/gems/railties-5.2.5/lib/rails/commands/server/server_command.rb:53:in `start'
         8: from ruby/2.7.0/gems/rack-2.2.7/lib/rack/server.rb:327:in `start'
         7: from ruby/2.7.0/gems/puma-6.3.0/lib/rack/handler/puma.rb:76:in `run'
         6: from ruby/2.7.0/gems/puma-6.3.0/lib/puma/launcher.rb:194:in `run'
         5: from ruby/2.7.0/gems/puma-6.3.0/lib/puma/single.rb:53:in `run'
         4: from ruby/2.7.0/gems/puma-6.3.0/lib/puma/server.rb:248:in `run'
         3: from ruby/2.7.0/gems/puma-6.3.0/lib/puma/server.rb:248:in `new'
         2: from ruby/2.7.0/gems/puma-6.3.0/lib/puma/reactor.rb:28:in `initialize'
         1: from ruby/2.7.0/gems/puma-6.3.0/lib/puma/reactor.rb:28:in `new'
ruby/2.7.0/gems/nio4r-2.5.2/lib/nio/selector.rb:17:in `initialize': unsupported backend:  (ArgumentError)

Working through the stack trace, I found that the Puma::Server.run method was not starting the :ruby reactor. Researching Puma, I found it version locked only to '~> 6'. It had been upgraded from 6.0.0 to 6.3.0. Reviewing the change log, I found that in Puma Change History, the breaking change was #2522.
>That occurred in 5.2.0, however the impact showed in the transition from 6.0.0 to 6.3.0.

Resolution is to upgrade nio4r using '~> 2'. Mine went to 2.5.9 and all was well.

Note: Change #2522, noted above, referred to NIL:Selector source code. That source code shows that the backends were expanded from the earlier [:ruby] to [:ruby, nil] in failing statement 17. Nil was, in fact, the value that was being chosen.