diff --git a/src/admin/user/commands.rs b/src/admin/user/commands.rs
index 9e4b348b..709e1f57 100644
--- a/src/admin/user/commands.rs
+++ b/src/admin/user/commands.rs
@@ -42,6 +42,12 @@ pub(super) async fn create(
 		return Ok(RoomMessageEventContent::text_plain(format!("Userid {user_id} already exists")));
 	}
 
+	if user_id.is_historical() {
+		return Ok(RoomMessageEventContent::text_plain(format!(
+			"User ID {user_id} does not conform to new Matrix identifier spec"
+		)));
+	}
+
 	let password = password.unwrap_or_else(|| utils::random_string(AUTO_GEN_PASSWORD_LENGTH));
 
 	// Create user