Approvers tried to get set but not defined

We are running dependabot-standalone against a private GitLab Core instance.

When an update is detected the MR is created, but the task fails when it tries to assign approvers to the MR.
This API endpoint is not available in Core.

My configuration is quite basic:

version: 2
updates:
  - package-ecosystem: composer
    directory: "/"
    schedule:
      interval: daily
      time: "19:00"
      timezone: Europe/Berlin
    commit-message:
      prefix: "dep"
    assignees:
      - lstoetzel

For some reason it tries to assign approvers, although no reviewers were defined.

I'm not too much into Ruby, but maybe reviewers hash should be a list of approvers and not a hash with an approvers key?

CI Log

[2021-01-04 15:42:15 +0000 tid=gnjwu0l7q] INFO: [lstoetzel/some-project=>composer] Fetching info for phpstan/phpstan-doctrine
[2021-01-04 15:42:19 +0000 tid=gnjwu0l7q] INFO: [lstoetzel/some-project=>composer] Update for phpstan/phpstan-doctrine  is impossible
[2021-01-04 15:42:19 +0000 tid=gnjwu0l7q] INFO: [lstoetzel/some-project=>composer] Fetching info for symfony/browser-kit
rake aborted!
Gitlab::Error::NotFound: Server responded with code 404, message: 404 Not Found. Request URI: https://private-gitlab-instance.de/api/v4/projects/lstoetzel%2Fsome-project/merge_requests/3/approvers
/home/dependabot/vendor/bundle/gems/gitlab-4.17.0/lib/gitlab/request.rb:59:in `validate'
/home/dependabot/vendor/bundle/gems/gitlab-4.17.0/lib/gitlab/request.rb:51:in `block (2 levels) in <class:Request>'
/home/dependabot/vendor/bundle/gems/gitlab-4.17.0/lib/gitlab/client/merge_request_approvals.rb:139:in `edit_merge_request_approvers'
/home/dependabot/vendor/bundle/gems/dependabot-common-0.129.0/lib/dependabot/clients/gitlab_with_retries.rb:67:in `public_send'
/home/dependabot/vendor/bundle/gems/dependabot-common-0.129.0/lib/dependabot/clients/gitlab_with_retries.rb:67:in `block in method_missing'
/home/dependabot/vendor/bundle/gems/dependabot-common-0.129.0/lib/dependabot/clients/gitlab_with_retries.rb:82:in `retry_connection_failures'
/home/dependabot/vendor/bundle/gems/dependabot-common-0.129.0/lib/dependabot/clients/gitlab_with_retries.rb:64:in `method_missing'
/home/dependabot/vendor/bundle/gems/dependabot-common-0.129.0/lib/dependabot/pull_request_creator/gitlab.rb:155:in `add_approvers_to_merge_request'
/home/dependabot/vendor/bundle/gems/dependabot-common-0.129.0/lib/dependabot/pull_request_creator/gitlab.rb:148:in `annotate_merge_request'
/home/dependabot/vendor/bundle/gems/dependabot-common-0.129.0/lib/dependabot/pull_request_creator/gitlab.rb:48:in `create'
/home/dependabot/vendor/bundle/gems/dependabot-common-0.129.0/lib/dependabot/pull_request_creator.rb:89:in `create'
/home/dependabot/app/services/gitlab/merge_request_creator.rb:36:in `call'
/home/dependabot/app/services/application_service.rb:7:in `call'
/home/dependabot/app/services/dependabot/merge_request_service.rb:37:in `create_mr'
/home/dependabot/app/services/dependabot/merge_request_service.rb:23:in `call'
/home/dependabot/app/services/application_service.rb:7:in `call'
/home/dependabot/app/services/dependency_updater.rb:154:in `create_mr'
/home/dependabot/app/services/dependency_updater.rb:90:in `block in update_dependencies'
/home/dependabot/app/services/dependency_updater.rb:87:in `each'
/home/dependabot/app/services/dependency_updater.rb:87:in `each_with_index'
/home/dependabot/app/services/dependency_updater.rb:87:in `update_dependencies'
/home/dependabot/app/services/dependency_updater.rb:14:in `call'
/home/dependabot/app/services/application_service.rb:7:in `call'
/home/dependabot/app/jobs/dependency_update_job.rb:17:in `perform'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/execution.rb:40:in `block in perform_now'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/callbacks.rb:112:in `block in run_callbacks'
/home/dependabot/vendor/bundle/gems/i18n-1.8.5/lib/i18n.rb:308:in `with_locale'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/translation.rb:9:in `block (2 levels) in <module:Translation>'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/callbacks.rb:121:in `instance_exec'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/callbacks.rb:121:in `block in run_callbacks'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/core_ext/time/zones.rb:66:in `use_zone'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/timezones.rb:9:in `block (2 levels) in <module:Timezones>'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/callbacks.rb:121:in `instance_exec'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/callbacks.rb:121:in `block in run_callbacks'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/logging.rb:25:in `block (4 levels) in <module:Logging>'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/notifications.rb:180:in `block in instrument'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/notifications/instrumenter.rb:24:in `instrument'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/notifications.rb:180:in `instrument'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/logging.rb:24:in `block (3 levels) in <module:Logging>'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/logging.rb:47:in `tag_logger'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/logging.rb:21:in `block (2 levels) in <module:Logging>'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/callbacks.rb:121:in `instance_exec'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/callbacks.rb:121:in `block in run_callbacks'
/home/dependabot/vendor/bundle/gems/activesupport-6.0.3.4/lib/active_support/callbacks.rb:139:in `run_callbacks'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/execution.rb:39:in `perform_now'
/home/dependabot/vendor/bundle/gems/activejob-6.0.3.4/lib/active_job/execution.rb:18:in `perform_now'
/home/dependabot/lib/tasks/dependabot.rake:6:in `block (2 levels) in <main>'
/home/dependabot/vendor/bundle/gems/rake-13.0.1/exe/rake:27:in `<top (required)>'
Tasks: TOP => dependabot:update
(See full trace by running task with --trace)
[2021-01-04 15:42:24 +0000 tid=gnjwu0l7q] INFO: [lstoetzel/some-project=>composer] Update for symfony/browser-kit  is impossible
[2021-01-04 15:42:24 +0000 tid=gnjwu0l7q] INFO: [lstoetzel/some-project=>composer] Updating following dependencies: doctrine/doctrine-bundle-2.2.2
[2021-01-04 15:42:24 +0000 tid=gnjwu0l7q] INFO: [lstoetzel/some-project=>composer] merge request !2 doesn't require rebasing
[2021-01-04 15:42:24 +0000 tid=gnjwu0l7q] INFO: [lstoetzel/some-project=>composer] Updating following dependencies: knplabs/knp-menu-bundle-3.1.0
Cleaning up file based variables
00:01
ERROR: Job failed: exit code 1

Admin message

Admin message

Approvers tried to get set but not defined