{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-20.09";

    home-manager = {
      url = "github:rycee/home-manager/master";
      inputs.nixpkgs.follows = "/nixpkgs";
    };

    secret = {
      type = "path";
      flake = false;
      path = "/home/u/nix/secret";
    };
  };

  outputs = { self, nixpkgs, home-manager, ... }: {
    nixosConfigurations.u = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        home-manager.nixosModules.home-manager
        ({ pkgs, ... }: {
          nix = {
            extraOptions = ''
              experimental-features = nix-command flakes
              print-missing = true
            '';
            package = pkgs.nixFlakes;
            registry.nixpkgs.flake = nixpkgs;
          };
        })
        ./configuration.nix
      ];
    };
  };
}

error: --- EvalError ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- nix
at: (262:28) in file: /nix/store/brdrs4v9hvvwvklgnc67l7vqmgd7fzl1-source/lib/modules.nix

   261|         value = builtins.addErrorContext (context name)
   262|           (args.${name} or config._module.args.${name});
      |                            ^
   263|       }) requiredArgs);

attribute 'secret' missing
(use '--show-trace' to show detailed location information)