diff --git a/activejob/lib/active_job/queue_adapters.rb b/activejob/lib/active_job/queue_adapters.rb
index 29f28963b4..b3d91dc562 100644
--- a/activejob/lib/active_job/queue_adapters.rb
+++ b/activejob/lib/active_job/queue_adapters.rb
@@ -50,6 +50,7 @@ module ActiveJob
     autoload :TestAdapter
 
     ADAPTER = 'Adapter'.freeze
+    private_constant :ADAPTER
 
     class << self
       def lookup(name)