diff --git a/test/pathname/test_pathname.rb b/test/pathname/test_pathname.rb
index daa32111bc..339ef7f487 100644
--- a/test/pathname/test_pathname.rb
+++ b/test/pathname/test_pathname.rb
@@ -1,3 +1,5 @@
+# -*- frozen_string_literal: true -*-
+
 require 'test/unit'
 require 'pathname'
 
@@ -189,8 +191,8 @@ class TestPathname < Test::Unit::TestCase
 
   if DOSISH
     defassert(:del_trailing_separator, "a", "a\\")
-    defassert(:del_trailing_separator, "\225\\".force_encoding("cp932"), "\225\\\\".force_encoding("cp932"))
-    defassert(:del_trailing_separator, "\225".force_encoding("cp437"), "\225\\\\".force_encoding("cp437"))
+    defassert(:del_trailing_separator, "\225\\".dup.force_encoding("cp932"), "\225\\\\".dup.force_encoding("cp932"))
+    defassert(:del_trailing_separator, "\225".dup.force_encoding("cp437"), "\225\\\\".dup.force_encoding("cp437"))
   end
 
   def test_plus
@@ -585,16 +587,16 @@ class TestPathname < Test::Unit::TestCase
     obj = Pathname.new("a"); assert_same(obj, obj.taint)
     obj = Pathname.new("a"); assert_same(obj, obj.untaint)
 
-    assert_equal(false, Pathname.new("a"      )           .tainted?)
-    assert_equal(false, Pathname.new("a"      )      .to_s.tainted?)
-    assert_equal(true,  Pathname.new("a"      ).taint     .tainted?)
-    assert_equal(true,  Pathname.new("a"      ).taint.to_s.tainted?)
-    assert_equal(true,  Pathname.new("a".taint)           .tainted?)
-    assert_equal(true,  Pathname.new("a".taint)      .to_s.tainted?)
-    assert_equal(true,  Pathname.new("a".taint).taint     .tainted?)
-    assert_equal(true,  Pathname.new("a".taint).taint.to_s.tainted?)
+    assert_equal(false, Pathname.new("a"          )           .tainted?)
+    assert_equal(false, Pathname.new("a"          )      .to_s.tainted?)
+    assert_equal(true,  Pathname.new("a"          ).taint     .tainted?)
+    assert_equal(true,  Pathname.new("a"          ).taint.to_s.tainted?)
+    assert_equal(true,  Pathname.new("a".dup.taint)           .tainted?)
+    assert_equal(true,  Pathname.new("a".dup.taint)      .to_s.tainted?)
+    assert_equal(true,  Pathname.new("a".dup.taint).taint     .tainted?)
+    assert_equal(true,  Pathname.new("a".dup.taint).taint.to_s.tainted?)
 
-    str = "a"
+    str = "a".dup
     path = Pathname.new(str)
     str.taint
     assert_equal(false, path     .tainted?)
@@ -607,7 +609,7 @@ class TestPathname < Test::Unit::TestCase
     assert_equal(false, Pathname.new("a").taint.untaint     .tainted?)
     assert_equal(false, Pathname.new("a").taint.untaint.to_s.tainted?)
 
-    str = "a".taint
+    str = "a".dup.taint
     path = Pathname.new(str)
     str.untaint
     assert_equal(true, path     .tainted?)