Accessing submodule options -- "used but not defined"

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1

I’ve been trying to modify an existing nixos module. It has some options bundled in a submodule. I tried to add an assertion on one of these, but I get an error like The option `services.foo.settings.my_option' is used but not defined.

The structure of the module is like

{ config, lib, pkgs, ... }:
let
  cfg = config.services.foo;
  settingsFormat = pkgs.formats.yaml { };
in
{
  options.services.foo = {
    enable = lib.mkEnableOption (lib.mdDoc "Foo");
    settings = lib.mkOption {
      type = lib.types.submodule {
        freeformType = settingsFormat.type;
        options.my_option = lib.mkOption {
          type = lib.types.str;
          example = "example.com";
          description = lib.mdDoc ''
            Foobar
          '';
        };
      };
      default = { };
      description = lib.mdDoc ''
        Foobar
      '';
    };
  };
  config = lib.mkIf cfg.enable {
    assertions = [
      {
        assertion = cfg.settings.my_option == null;
        message = "Can't be null";
      }
    ];
  };
}

Is there some special way to access the options in the submodule?

2

That error message tells you, that you are reading from an option you haven’t written to.

This means that the option neither has a default, nor has been set explicitely by the user.

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