I just ran into this myself and it was because my extra-builtins.nix wasn’t known to git. Once I added it and did a rebuild it worked.
Figured this out by verifying that the file was in the extra-builtins-file config with nix config show.
Looked up the documentation on that config option and it says that if the file doesn’t exist at the path given then it’ll set to null. Checked the path it was trying to load (which was in my store) and in fact it wasn’t there. Had the idea maybe it wasn’t in git, it wasn’t…tried it out.