Upgrading old application from Ruby 1.8.7 to 2.7.4, getting error about RUBY_ENGINE in ffi

I'm getting an error about RUBY_ENGINE when I try to use rb-inotify.

/usr/local/rvm/gems/ruby-2.7.4/gems/ffi-1.15.3/lib/ffi.rb: uninitialized constant RUBY_ENGINE (NameError)