Please see https://github.com/mogman1/protobuf_error_script for a test script that produces this bug reliably and the issue submitted to JRuby at jruby/jruby#6225. As I was able to pinpoint this to a specific commit in JRuby (see issue), I believe this to be a JRuby bug, but thought I should also log an issue here so you are aware of it.
Please see https://github.com/mogman1/protobuf_error_script for a test script that produces this bug reliably and the issue submitted to JRuby at jruby/jruby#6225. As I was able to pinpoint this to a specific commit in JRuby (see issue), I believe this to be a JRuby bug, but thought I should also log an issue here so you are aware of it.