Adding custom public feeds in dependabot.yml

I’m trying to get Dependabot to use our nightly NuGet feed for our OSS project, but the dependabot.yml validator rejected this configuration:

version: 2
registries:
  akka-nightly:
    type: nuget-feed
    url: https://f.feedz.io/akkadotnet/akka/nuget/index.json
updates:
- package-ecosystem: nuget
  directory: "/"
  registries:
    - akka-nightly
  schedule:
    interval: daily
  open-pull-requests-limit: 10

Our public NuGet feed requires no authentication of any kind - how can we configure Dependabot to use it?