diff --git a/lib/utils/reserved-users.js b/lib/utils/reserved-users.js
--- a/lib/utils/reserved-users.js
+++ b/lib/utils/reserved-users.js
@@ -1,6 +1,7 @@
 // @flow
 
-// All entries in this list must be lowercase!!
+// All entries in this list must be lowercase and must also be included in
+// `services/identity/reserved_usernames.json`!!
 const reservedUsernamesSet: $ReadOnlySet<string> = new Set([
   // Additional Reserved Keywords
   'comm',
diff --git a/services/identity/src/config.rs b/services/identity/src/config.rs
--- a/services/identity/src/config.rs
+++ b/services/identity/src/config.rs
@@ -75,6 +75,8 @@
 }
 
 fn get_reserved_usernames_set() -> Result<HashSet<String>, Error> {
+  // All entries in `reserved_usernames.json` must be lowercase and must also be
+  // included in `lib/utils/reserved-users.js`!!
   let contents = include_str!("../reserved_usernames.json");
   let reserved_usernames: Vec<String> = serde_json::from_str(contents)?;