diff --git a/test/stringio/test_ractor.rb b/test/stringio/test_ractor.rb
index 1c334e2c..4a2033bc 100644
--- a/test/stringio/test_ractor.rb
+++ b/test/stringio/test_ractor.rb
@@ -11,7 +11,7 @@ def test_ractor
       require "stringio"
       $VERBOSE = nil
       r = Ractor.new do
-        io = StringIO.new("")
+        io = StringIO.new(+"")
         io.puts "abc"
         io.truncate(0)
         io.puts "def"