diff --git a/ChangeLog b/ChangeLog
index c41a9689f8..797a879c39 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Wed Dec 30 09:20:00 2015  Yuki Kurihara  <co000ri@gmail.com>
+
+	* test/stringio/test_io.rb (test_flag): add assertion for error when
+	  text and binary mode are mixed.
+	  [ruby-dev:49465] [Feature #11921]
+
 Wed Dec 30 08:43:59 2015  Yuki Kurihara  <co000ri@gmail.com>
 
 	* test/stringio/test_stringio.rb (test_initialize): add test for
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 2b79acea09..33a2ab883d 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1906,6 +1906,10 @@ class TestIO < Test::Unit::TestCase
       assert_raise(ArgumentError) do
         open(t.path, "rr") { }
       end
+
+      assert_raise(ArgumentError) do
+        open(t.path, "rbt") { }
+      end
     }
   end