Can't have custom domains on GitLab pages with only numbers in the unqualified hostname

Summary

I would like to publish a website in a custom subdomain, but the subdomain should be 2017.mywebsite.tld. I tried to configure it in GitLab pages, but GitLab refused to accept it. This only happens if the numeric-only section of the domain is the first one (www.2017.mywebsite.tld works fine).

Steps to reproduce

1. Create a new repository with GitLab pages enabled
2. Go to the pages settings
3. Click the "Add domain" button
4. Enter "2017.example.com" as the domain
5. Submit the form

What is the current bug behavior?

The error message "Domain unqualified hostname part cannot consist of numeric values only" appears.

What is the expected correct behavior?

The domain should be added. I also tested it on a dummy GitHub repository, and they allow to use subdomains like that.

Output of checks

This bug happens on GitLab.com