Operation not permitted on chmod post-receive

Created by: kenips

I get this error when gitlab tries to create / update project:

Errno::EPERM (Operation not permitted - /home/git/repositories/project.git/hooks/post-receive):
  app/models/project/repository_trait.rb:51:in `chmod'
  app/models/project/repository_trait.rb:51:in `write_hook'
  app/models/project/repository_trait.rb:36:in `block in write_hooks'
  app/models/project/repository_trait.rb:35:in `each'
  app/models/project/repository_trait.rb:35:in `write_hooks'
  app/models/project/repository_trait.rb:78:in `update_repository'
  app/models/project_observer.rb:3:in `after_save'
  app/controllers/projects_controller.rb:60:in `block in update'
  app/controllers/projects_controller.rb:59:in `update'

Running diagnostic tells me that everything is fine and I'm sure that the user gitlab run as (me) can write to the folder

# bundle exec rake gitlab:app:status RAILS_ENV=production
Starting diagnostic
config/database.yml............exists
config/gitlab.yml............exists
/home/git/repositories/............exists
/home/git/repositories/ is writable?............YES
remote: Counting objects: 320, done.
remote: Compressing objects: 100% (251/251), done.
Receiving objects: 100% (320/320), 30.48 KiB, done.
Resolving deltas: 100% (93/93), done.
remote: Total 320 (delta 93), reused 0 (delta 0)
Can clone gitolite-admin?............YES
UMASK for .gitolite.rc is 0007? ............YES

Finished

If I manually run chmod I get the same result, since I am only part of the group 'git' and not owner of the file.

# chmod 775 /home/git/repositories/project.git/hooks/post-receive
chmod: changing permissions of `/home/git/repositories/project.git/hooks/post-receive': Operation not permitted

So am I missing something? Or should gitlab not be running chmod as me?