Rugged::ReferenceError: Reference 'refs/heads/master' not found

GitLab 8.6.1

from app/models/repository.rb:875:in `head' from app/models/repository.rb:875:in `main_language' from app/services/git_push_service.rb:57:in `update_main_language' from app/services/git_push_service.rb:48:in `execute' from app/workers/post_receive.rb:44:in `block in process_project_changes' from app/workers/post_receive.rb:31:in `each' from app/workers/post_receive.rb:31:in `process_project_changes' from app/workers/post_receive.rb:23:in `perform' from sidekiq/processor.rb:150:in `execute_job' from sidekiq/processor.rb:132:in `block (2 levels) in process' from sidekiq/middleware/chain.rb:127:in `block in invoke' from lib/gitlab/sidekiq_middleware/memory_killer.rb:17:in `call' from sidekiq/middleware/chain.rb:129:in `block in invoke' from sidekiq/middleware/server/active_record.rb:6:in `call' from sidekiq/middleware/chain.rb:129:in `block in invoke' from sidekiq/middleware/server/retry_jobs.rb:74:in `call' from sidekiq/middleware/chain.rb:129:in `block in invoke' from sidekiq/middleware/server/logging.rb:11:in `block in call' from sidekiq/logging.rb:30:in `with_context' from sidekiq/middleware/server/logging.rb:7:in `call' from sidekiq/middleware/chain.rb:129:in `block in invoke' from sidekiq/middleware/chain.rb:132:in `call' from sidekiq/middleware/chain.rb:132:in `invoke' from sidekiq/processor.rb:127:in `block in process' from sidekiq/processor.rb:166:in `stats' from sidekiq/processor.rb:126:in `process' from sidekiq/processor.rb:79:in `process_one' from sidekiq/processor.rb:67:in `run' from sidekiq/util.rb:16:in `watchdog' from sidekiq/util.rb:24:in `block in safe_thread'