update:

examples/example-system.flake.nix

@@ -27,19 +27,23 @@
     };
   };
 
-  outputs = { self, nixpkgs, linger, pihole, ... }:
-    let
+  outputs = {
+    self,
+    nixpkgs,
+    linger,
+    pihole,
+    ...
+  }: let
     system = "x86_64-linux";
     # use x86_64 packages from nixpkgs
     pkgs = nixpkgs.legacyPackages.${system};
-
   in {
     nixosConfigurations."nixos-example-system" = nixpkgs.lib.nixosSystem {
       # nixosSystem needs to know the system architecture
       inherit system;
       modules = [
         # a small module for enabling nix flakes
-          { ... }: {
+        ({...}: {
           nix = {
             packge = pkgs.nixFlakes;
             extraOptions = "experimental-features = nix-command flake";
@@ -48,7 +52,7 @@
             # see https://dataswamp.org/~solene/2022-07-20-nixos-flakes-command-sync-with-system.html
             registry.nixpkgs.flake = nixpkgs;
           };
-          }
+        })
 
         # some existing system & hardware configuration modules; it is assumed that a user named `pihole` is defined here
         # and that the user has sub-uids/gids configured (e.g. via the `users.users.pihole.subUidRanges/subGidRanges` options)
@@ -60,7 +64,7 @@
         pihole.nixosModules.${system}.default
 
         # in another module we can now configure the lingering behaviour (could also be part of ./configuration.nix)
-          { ... }: {
+        ({...}: {
           # required for stable restarts of the Pi-hole container (try to remove it to see the warning from the pihole-flake)
           boot.cleanTmpDir = true;
 
@@ -95,10 +99,10 @@
           # we need to open the ports in the firewall to make the service accessible beyond `localhost`
           # assuming that Pi-hole is exposed on the host interface `eth0`
           networking.firewall.interfaces.eth0 = {
-              allowedTCPPorts = [ 5335 8080 ];
-              allowedUDPPorts = [ 5335 ];
+            allowedTCPPorts = [5335 8080];
+            allowedUDPPorts = [5335];
           };
-          }
+        })
       ];
     };
   };

modules/pihole-container.factory.nix

@@ -327,7 +327,7 @@ in rec {
   config = mkIf cfg.enable {
 
     assertions = [
-      { assertion = length hostUserCfg.subUidRanges > 0 && length hostUserCfg.subGidRanges > 0;
+      { assertion = length hostUserCfg.subUidRanges > 0 && length hostUserCfg.subGidRanges > 0 || hostUserCfg.autoSubUidGidRanges;
         message = ''
           The host user most have configured subUidRanges & subGidRanges as pihole is running in a rootless podman container.
         '';