diff --git a/ChangeLog b/ChangeLog
index 699e5e69a8..e6434a8ea4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Oct 13 19:41:32 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (copy_stream_body): use IO to write to copy to duplex IO.
+	  http://twitter.com/knu/status/786505317974585344
+
 Thu Oct 13 17:05:57 2016  Dwain Faithfull  <dwfaithfull@gmail.com>
 
 	* hash.c (rb_hash_compact, rb_hash_compact_bang): Removes nil
diff --git a/io.c b/io.c
index 35f6fb699b..cba9318590 100644
--- a/io.c
+++ b/io.c
@@ -10657,7 +10657,7 @@ copy_stream_body(VALUE arg)
     else {
 	VALUE tmp_io = rb_io_check_io(dst_io);
 	if (!NIL_P(tmp_io)) {
-	    dst_io = tmp_io;
+	    dst_io = GetWriteIO(tmp_io);
 	}
 	else if (!RB_TYPE_P(dst_io, T_FILE)) {
 	    VALUE args[3];
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index c188f2cda3..fa3b3578db 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1105,6 +1105,18 @@ class TestIO < Test::Unit::TestCase
     }
   end
 
+  def test_copy_stream_to_duplex_io
+    result = IO.pipe {|a,w|
+      Thread.start {w.puts "yes"; w.close}
+      IO.popen([EnvUtil.rubybin, '-pe$_="#$.:#$_"'], "r+") {|b|
+        IO.copy_stream(a, b)
+        b.close_write
+        b.read
+      }
+    }
+    assert_equal("1:yes\n", result)
+  end
+
   def ruby(*args)
     args = ['-e', '$>.write($<.read)'] if args.empty?
     ruby = EnvUtil.rubybin