I want to host the repository https://github.com/username/username.github.io on github pages and use the custom subdomain user.institution.ac.in. I added a CNAME record for username.github.io in my domain's DNS settings, but when I tried to add a CNAME record for the subdomain in the respository, it shows: "The CNAME user.institution.ac.in is already taken". There are no other repositories using this CNAME currently, so I can't figure out the problem. Please help.