diff --git a/test/-ext-/string/test_modify_expand.rb b/test/-ext-/string/test_modify_expand.rb
index d3f5a17037..c7093e3892 100644
--- a/test/-ext-/string/test_modify_expand.rb
+++ b/test/-ext-/string/test_modify_expand.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: false
 require 'test/unit'
 require "-test-/string"
+require "rbconfig/sizeof"
 
 class Test_StringModifyExpand < Test::Unit::TestCase
   def test_modify_expand_memory_leak
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index c83e8c9ec4..a3f6dfc3e8 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: false
 require 'test/unit'
 require 'stringio'
+require "rbconfig/sizeof"
 require_relative '../ruby/ut_eof'
 
 class TestStringIO < Test::Unit::TestCase