hiera lookup failure

Given this /etc/puppetlabs/code/hiera.yaml --- :backends: - json :hierarchy: - "%{hostname}.%{nt_environment}.%{nt_location}" - "%{nt_type}.%{nt_environment}.%{nt_location}" - "%{nt_type}.%{nt_environment}" - "%{nt_environment}.%{nt_location}" - "%{nt_environment}" - "%{nt_location}" - world - default :logger: console # debug :json: :datadir: '/etc/puppetlabs/code/environments/%{::environment}/hieradata' # this is the default anyway and there exists /etc/puppetlabs/code/environments/prod/hieradata/sac.json, this "puppet lookup --debug icinga_servers --environment prod" produces Debug: hiera(): Looking for data source puppetmaster.ops Debug: hiera(): Looking for data source ops.sac Debug: hiera(): Looking for data source ops Debug: hiera(): Looking for data source sac Debug: hiera(): Looking for data source world Debug: hiera(): Looking for data source default (& lots more of course).
Why can't it find the /etc/puppetlabs/code/environments/prod/hieradata/sac.json file which contains the key I'm looking for ?
Puppet server 2.4