Yea, this is a bug in Nix 2.18: error: path is not valid using nix ==2.18.0 · Issue #9052 · NixOS/nix · GitHub
Only nixUnstable has been updated to 2.18 though, so you if you set nix.package = pkgs.nixUnstable, that’s why you’re experiencing this. Shouldn’t happen if you remove that and let it use the default Nix version.