diff --git a/constants/stdout.rb b/constants/stdout.rb
index 594ebcc..d6e9efc 100644
--- a/constants/stdout.rb
+++ b/constants/stdout.rb
@@ -1,3 +1,5 @@
+require 'stringio'
+
 $stdout = StringIO.new
 
 puts 'Hello, world!'
diff --git a/constants/stdout_puts.rb b/constants/stdout_puts.rb
index b171c1c..0064d73 100644
--- a/constants/stdout_puts.rb
+++ b/constants/stdout_puts.rb
@@ -1,3 +1,5 @@
+require 'stringio'
+
 $stdout = StringIO.new
 
 puts 'Hello, world!'
diff --git a/variables/stdout.rb b/variables/stdout.rb
index 9dc6972..077a3a5 100644
--- a/variables/stdout.rb
+++ b/variables/stdout.rb
@@ -1,3 +1,5 @@
+require 'stringio'
+
 $stdout = StringIO.new
 
 puts 'Hello, world!'