diff --git a/test/test-html.rb b/test/test-html.rb
index 3e036ff..07ce718 100644
--- a/test/test-html.rb
+++ b/test/test-html.rb
@@ -1,4 +1,4 @@
-# Copyright (C) 2013-2017 Kouhei Sutou
+# Copyright (C) 2013-2024 Sutou Kouhei
#
# This library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
@@ -188,7 +188,7 @@ def normalize_decomposed_data(decomposed_data)
sub_test_case("detect") do
def test_nothing
- @data.body = <<-HTML.force_encoding("UTF-8")
+ @data.body = <<-HTML.dup.force_encoding("UTF-8")
Hello