Adding release-groups to the end of this url breaks it

http://test.musicbrainz.org/ws/2/release/8f1cf394-b4d5-460b-add2-ca696984243a?inc=recordings+artists+labels+puids+media+discids+artist-credits+artist-rels+label-rels+recording-rels+release-group-rels+url-rels+work-rels+release-groups

Giving error

Caught exception in MusicBrainz::Server::Controller::WS::2->release "Can't call method "names" on an undefined value at /home/mbserver/musicbrainz/mb_server/script/../lib/MusicBrainz/Server/WebService/XMLSerializer.pm line 135."

The url works ok without release-groups, and also works with release-groups included but other incs not included.