Unable to create new wiki pages

Hi, i can't create new wiki pages in some of my gitlab projects.

I got the following error from the log (gitlab 8.7.0, omnibus installation)

NoMethodError (undefined method `username' for #<Group:0x00000004d6a728>):
 app/models/project_wiki.rb:158:in `default_message'
 app/models/project_wiki.rb:152:in `commit_details'
 app/models/project_wiki.rb:91:in `create_page'
 app/models/wiki_page.rb:183:in `save'
 app/models/wiki_page.rb:143:in `create'
 app/services/wiki_pages/create_service.rb:6:in `execute'
 app/controllers/projects/wikis_controller.rb:58:in `create'
 lib/gitlab/middleware/go.rb:16:in `call