Can't mirror from public GitHub repository

I created a new repository called "mirror-test", which has no commits in its repository. I go to its Settings -> Repository page and expand the "Mirroring repositories" section. I enter the URL https://github.com/kenahoo/dotfiles.git in the "Git repository URL" box, and try to select "Pull" as the direction, but the dropdown menu for it is greyed out:

How do I get that option enabled?

I'm using GitLab Enterprise Edition 12.1.6-ee.