diff --git a/benchmarks/table-regexps.yml b/benchmarks/table-regexps.yml
index 446e7e57..a701f8c6 100644
--- a/benchmarks/table-regexps.yml
+++ b/benchmarks/table-regexps.yml
@@ -1,6 +1,6 @@
 prelude: |
   require "json"
-  require "set"
+  require "set" unless defined?(::Set)
 
   all_codepoints = (0..0x10ffff).map{_1.chr("UTF-8") rescue nil}.compact
 
diff --git a/lib/net/imap/sequence_set.rb b/lib/net/imap/sequence_set.rb
index 40b02dae..7e5206a1 100644
--- a/lib/net/imap/sequence_set.rb
+++ b/lib/net/imap/sequence_set.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "set" unless defined?(::Set)
+
 module Net
   class IMAP
 
diff --git a/rakelib/string_prep_tables_generator.rb b/rakelib/string_prep_tables_generator.rb
index d1b4443d..59028a69 100644
--- a/rakelib/string_prep_tables_generator.rb
+++ b/rakelib/string_prep_tables_generator.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "set" unless defined?(::Set)
+
 # Generator for stringprep regexps.
 #
 # Combines Unicode character classes with generated tables.  Generated regexps
diff --git a/test/net/imap/test_sequence_set.rb b/test/net/imap/test_sequence_set.rb
index 903eea0a..e8460139 100644
--- a/test/net/imap/test_sequence_set.rb
+++ b/test/net/imap/test_sequence_set.rb
@@ -2,7 +2,6 @@
 
 require "net/imap"
 require "test/unit"
-require "set"
 
 class IMAPSequenceSetTest < Test::Unit::TestCase
   # alias for convenience
diff --git a/test/net/imap/test_stringprep_tables.rb b/test/net/imap/test_stringprep_tables.rb
index 3f9d22f1..b87d1efe 100644
--- a/test/net/imap/test_stringprep_tables.rb
+++ b/test/net/imap/test_stringprep_tables.rb
@@ -3,7 +3,6 @@
 require "net/imap"
 require "test/unit"
 require "json"
-require "set"
 
 require_relative "../../../rakelib/string_prep_tables_generator"