diff --git a/README.md b/README.md
index f867d6f..ffc5519 100644
--- a/README.md
+++ b/README.md
@@ -63,7 +63,7 @@ Defaults are set in `flake.nix` and used across modules. Update them there:
 | `time.timeZone` | `America/New_York` | `modules/common.nix` |
 | `i18n.defaultLocale` | `en_US.UTF-8` | `modules/common.nix` |
 
-Also rename the default `hosts/atlas/` directory to match your actual hostname, and ensure the same hostname is set in `flake.nix` (the flake uses it to locate `hosts/<hostname>/hardware-configuration.nix`).
+Rename the default `hosts/atlas/` directory to match your hostname (e.g., `hosts/desktop/`), then update the `hostname` value in `flake.nix` to match. The flake uses it to locate `hosts/<hostname>/hardware-configuration.nix`.
 
 ### 3. Stage Files in Git
 
diff --git a/modules/common.nix b/modules/common.nix
index e83d742..96aaf14 100644
--- a/modules/common.nix
+++ b/modules/common.nix
@@ -3,7 +3,7 @@
 {
   assertions = [
     {
-      assertion = builtins.match "^[a-z_][a-z0-9_-]*$" username != null;
+      assertion = builtins.match "^[a-z_][a-z0-9_]*$" username != null;
       message = "username must be a simple system user name without path separators.";
     }
   ];