diff --git a/ChangeLog b/ChangeLog
index 64071007c5..c649166b95 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Feb 15 17:41:31 2012  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* io.c (io_strsetbuf): call rb_str_modify to make str independent
+	  before calling rb_str_set_len for r34580.
+
 Wed Feb 15 12:30:10 2012  Eric Hodel  <drbrain@segment7.net>
 
 	* ext/zlib/zlib.c (Init_zlib):  Added Zlib::TEXT and note that
diff --git a/io.c b/io.c
index 074d2c50f7..f404b539a4 100644
--- a/io.c
+++ b/io.c
@@ -2084,7 +2084,10 @@ io_setstrbuf(VALUE *str, long len)
 	VALUE s = StringValue(*str);
 	long clen = RSTRING_LEN(s);
 	if (clen >= len) {
-	    if (clen != len) rb_str_set_len(s, len);
+	    if (clen != len) {
+		rb_str_modify(s);
+		rb_str_set_len(s, len);
+	    }
 	    return;
 	}
 	len -= clen;