JRuby 10.0.0.0 doesn't pass the CI for now. https://github.com/mongomapper/plucky/pull/59/commits/a3e023798c51a7fecb73be8d2a4c6d991f8e1e7d This problem has been reported at https://jira.mongodb.org/browse/RUBY-3650 as mongo gem's issue.