Prometheus manifest breaks puppet on brand new nodes

I have just seen this error while trying to run Puppet for the first time on a brand new VM:

Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Operator '[]' is not applicable to an Undef Value. at /etc/puppet/code/environments/production/site-modules/profile/manifests/prometheus/apt_config.pp:3:6 on node logstash0.internal.softwareheritage.org

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Migrated from T1162 (view on Phabricator)