diff --git a/lib/concurrent/atomic/mutex_atomic_boolean.rb b/lib/concurrent/atomic/mutex_atomic_boolean.rb
index ffa3e365c..a033de4ca 100644
--- a/lib/concurrent/atomic/mutex_atomic_boolean.rb
+++ b/lib/concurrent/atomic/mutex_atomic_boolean.rb
@@ -50,6 +50,8 @@ def ns_initialize(initial)
       @value = !!initial
     end
 
+    private
+
     # @!visibility private
     def ns_make_value(value)
       old = @value