diff --git a/lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb b/lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb
index de39abaab..24d039b2f 100644
--- a/lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb
+++ b/lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb
@@ -42,7 +42,7 @@ def self.make_synchronized_on_rbx(klass)
           private
 
           def _mon_initialize
-            @_monitor = Monitor.new unless @_monitor # avoid double initialisation
+            @_monitor ||= Monitor.new # avoid double initialisation
           end
 
           def self.new(*args)