Can't install any plugins on 5.0.0-alpha3

Whenever I run ./bin/logstash-plugin, I keep getting error: Error opening script file: /.../Downloads/logstash-5.0.0-alpha3/lib/pluginmanager/main.rb (No such file or directory), even though if I cd or cat to the path it works perfectly fine. I am using cygwin, if it matters. any help would be appreciated.