diff --git a/guides/source/rails_on_rack.md b/guides/source/rails_on_rack.md
index 1fac6d9883..ac355b4a08 100644
--- a/guides/source/rails_on_rack.md
+++ b/guides/source/rails_on_rack.md
@@ -229,7 +229,7 @@ Much of Action Controller's functionality is implemented as Middlewares. The fol
 
  **`Rack::Lock`**
 
-* Sets `env["rack.multithread"]` flag to `true` and wraps the application within a Mutex.
+* Sets `env["rack.multithread"]` flag to `false` and wraps the application within a Mutex.
 
  **`ActiveSupport::Cache::Strategy::LocalCache::Middleware`**