Create CI/CD project never succeeds, preventing mirroring

Creating a CI/CD project hangs forever and mirroring doesn't work.

2018-03-09_14:43:31.74000 2018-03-09T14:43:31.739Z 462 TID-ounur4ewg WARN: {"context":"Job raised exception","job":{"class":"Gitlab::GithubImport::Stage::FinishImportWorker","args":[12],"retry":5,"queue":"github_importer:github_import_stage_finish_import","queue_namespace":"github_importer","dead":false,"jid":"3e2a89b52a8363e5678b7500","created_at":1520606436.1329708,"enqueued_at":1520606611.7159686,"error_message":"can't convert nil into an exact number","error_class":"TypeError","failed_at":1520606436.1627452,"retry_count":2,"retried_at":1520606543.9957519},"jobstr":"{\"class\":\"Gitlab::GithubImport::Stage::FinishImportWorker\",\"args\":[12],\"retry\":5,\"queue\":\"github_importer:github_import_stage_finish_import\",\"queue_namespace\":\"github_importer\",\"dead\":false,\"jid\":\"3e2a89b52a8363e5678b7500\",\"created_at\":1520606436.1329708,\"enqueued_at\":1520606611.7159686,\"error_message\":\"can't convert nil into an exact number\",\"error_class\":\"TypeError\",\"failed_at\":1520606436.1627452,\"retry_count\":2,\"retried_at\":1520606543.9957519}"}
2018-03-09_14:43:31.74028 2018-03-09T14:43:31.740Z 462 TID-ounur4ewg WARN: TypeError: can't convert nil into an exact number
2018-03-09_14:43:31.74056 2018-03-09T14:43:31.740Z 462 TID-ounur4ewg WARN: /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activesupport-4.2.10/lib/active_support/core_ext/time/calculations.rb:240:in `-'
2018-03-09_14:43:31.74058 /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activesupport-4.2.10/lib/active_support/core_ext/time/calculations.rb:240:in `minus_with_duration'
2018-03-09_14:43:31.74058 /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activesupport-4.2.10/lib/active_support/core_ext/time/calculations.rb:251:in `minus_with_coercion'
2018-03-09_14:43:31.74059 /opt/gitlab/embedded/service/gitlab-rails/ee/app/models/project_mirror_data.rb:42:in `base_delay'
2018-03-09_14:43:31.74059 /opt/gitlab/embedded/service/gitlab-rails/ee/app/models/project_mirror_data.rb:25:in `set_next_execution_timestamp'