See also bug 33960. Found it testing there. [2023/11/03 10:33:00] [ERROR] GET /api/v1/biblios: unhandled exception (Mojo::Exception)<<Can't call method "to_api" on an undefined value at /usr/share/koha/Koha/Biblio.pm line 1433.>> => my $json_biblioitem = $self->biblioitem->to_api( $args ); Just testing /api/v1/biblios with Accept=application/json. Note that Accept=application/marc works fine.