500 error when setting up repository mirroring from GitHub -> GitLab.com

Summary

GitLab.com throws a 500 when I set up repository mirroring from GitHub to GitLab.com.

Steps to reproduce

1. Import a project from GitHub
   1. New project > Import project > Import project from > GitHub
   2. Click "Import" next to the repository to be imported
   3. Wait for the import to complete successfully
2. In the new GitLab.com project's page, set up pull mirroring from GitHub
   1. Settings > Repository > Mirroring repositories
   2. Enter a URL like ssh://github.com:nfriend/qmk_firmware.git
   3. Set Mirror direction = Pull
   4. Set Authentication method = SSH public key
   5. Add the public key that is generated to GitHub
      1. GitHub.com -> Settings -> SSH and GPG keys -> New SSH key
   6. Click "Mirror Repository"

Example Project

Here is the project I am trying to mirror from GitHub: https://github.com/nfriend/qmk_firmware

Here is the project that GitLab.com imported for me: https://gitlab.com/nfriend/qmk_firmware

What is the current bug behavior?

The page shows a 500 error very quickly after I click the "Mirror Repository" button.

What is the expected correct behavior?

The page should reload correctly (and show me a friendly error message if I'm doing something wrong).

Output of checks

This bug happens on GitLab.com.