1 files changed, 9 insertions, 3 deletions
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 42149ac32a..14592e4d58 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -3316,11 +3316,17 @@ __END__
     data = "a" * 100
     with_pipe do |r,w|
       th = Thread.new {r.sysread(100, buf)}
+
       Thread.pass until th.stop?
-      buf.replace("")
-      assert_empty(buf, bug6099)
+
+      assert_equal 100, buf.bytesize
+
+      msg = /can't modify string; temporarily locked/
+      assert_raise_with_message(RuntimeError, msg) do
+        buf.replace("")
+      end
+      assert_predicate(th, :alive?)
       w.write(data)
-      Thread.pass while th.alive?
       th.join
     end
     assert_equal(data, buf, bug6099)