diff --git a/lib/sidekiq/delay.rb b/lib/sidekiq/delay.rb
index 8abfaff7..60bdb408 100644
--- a/lib/sidekiq/delay.rb
+++ b/lib/sidekiq/delay.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Sidekiq
   module Extensions
 
diff --git a/lib/sidekiq/job_logger.rb b/lib/sidekiq/job_logger.rb
index 5e7fd7f8..ec9764bf 100644
--- a/lib/sidekiq/job_logger.rb
+++ b/lib/sidekiq/job_logger.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Sidekiq
   class JobLogger
 
diff --git a/lib/sidekiq/job_retry.rb b/lib/sidekiq/job_retry.rb
index 6e88c984..f2156a32 100644
--- a/lib/sidekiq/job_retry.rb
+++ b/lib/sidekiq/job_retry.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'sidekiq/scheduled'
 require 'sidekiq/api'
 
diff --git a/lib/sidekiq/middleware/server/active_record.rb b/lib/sidekiq/middleware/server/active_record.rb
index 0ef2aab7..f6e7ca36 100644
--- a/lib/sidekiq/middleware/server/active_record.rb
+++ b/lib/sidekiq/middleware/server/active_record.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Sidekiq
   module Middleware
     module Server