diff --git a/src/command.ts b/src/command.ts
index 2ffb141..83dde34 100644
--- a/src/command.ts
+++ b/src/command.ts
@@ -19,7 +19,7 @@ function unmask_room_name(name: string) {
     }
 
     if (name.startsWith('costum-room#')) {
-        return name.substring(12);
+        return name.substring('costum-room#'.length);
     }
 
     return null;