diff --git a/Gemfile b/Gemfile
index eece3227..8a72bcae 100644
--- a/Gemfile
+++ b/Gemfile
@@ -83,7 +83,6 @@ group :test do
   gem 'm', '~> 1.5'
   gem 'pry', '>= 0.10'
   gem 'byebug', '~> 8.2' if RUBY_VERSION < '2.2'
-  gem 'pry-byebug', platforms: :ruby
 end
 
 group :development, :test do
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'