diff --git a/test/fiddle/test_func.rb b/test/fiddle/test_func.rb
index 8f7b0c2874..8517965223 100644
--- a/test/fiddle/test_func.rb
+++ b/test/fiddle/test_func.rb
@@ -38,7 +38,7 @@ module Fiddle
     def test_string
       stress, GC.stress = GC.stress, true
       f = Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
-      buff = "000"
+      buff = +"000"
       str = f.call(buff, "123")
       assert_equal("123", buff)
       assert_equal("123", str.to_s)
diff --git a/test/fiddle/test_function.rb b/test/fiddle/test_function.rb
index 477b6a86dc..cbf84eae9d 100644
--- a/test/fiddle/test_function.rb
+++ b/test/fiddle/test_function.rb
@@ -60,13 +60,13 @@ module Fiddle
       func = Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
 
       assert_nil Fiddle.last_error
-      func.call("000", "123")
+      func.call(+"000", "123")
       refute_nil Fiddle.last_error
     end
 
     def test_strcpy
       f = Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
-      buff = "000"
+      buff = +"000"
       str = f.call(buff, "123")
       assert_equal("123", buff)
       assert_equal("123", str.to_s)
diff --git a/test/fiddle/test_import.rb b/test/fiddle/test_import.rb
index 0cd5e76a80..ff16d17d50 100644
--- a/test/fiddle/test_import.rb
+++ b/test/fiddle/test_import.rb
@@ -128,7 +128,7 @@ module Fiddle
     end
 
     def test_strcpy()
-      buff = "000"
+      buff = +"000"
       str = LIBC.strcpy(buff, "123")
       assert_equal("123", buff)
       assert_equal("123", str.to_s)