No tooltip shown for project's visibility level info

My English is poor，so I just describe the problem by pictures.

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The location of the problem

The tag attributes (class) and the Browser > F12 > Elements (the title-attribute is blank)

Code

the code path is gitlab/app/views/projects/_home_panel.html.haml

and the function visibility_icon_description and it's path is gitlab/app/helpers/visibility_level_helper.rb

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I think the problem may come from the differences between the parameter @project and the red case Project. I do %p= @project in HAML and it printed #<Project:0x00000000433dcfa0>, but I don't what is the red case Project.