diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index e3f973a3..0533fdac 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'thread_safe'
 require 'jsonapi/include_directive'
 require 'active_model/serializer/collection_serializer'
 require 'active_model/serializer/array_serializer'