Custom domain taken after renaming repository

I had a repository published to github pages that was not using the root level name [organization].github.io (it was published to [organization].github.io/[repository]), and it used a custom domain name. I renamed the repository to [organization].github.io, which cleared the custom domain field, and now I cannot use that custom domain; github says “The custom domain ______ is already taken.”

Any ideas on what to do here? Does it just need time to clear out cache or something?