diff --git a/ChangeLog b/ChangeLog
index 8b3224034e..d57dbcc744 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Fri Dec 18 17:24:09 2015  Koichi Sasada  <ko1@atdot.net>
+
+	* compile.c (ibf_load_object_string): use fstring if frozen string.
+
 Fri Dec 18 16:54:38 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/stringio/stringio.c (strio_set_encoding): add StringIO's own
diff --git a/compile.c b/compile.c
index f858fbb681..ed6f9686cf 100644
--- a/compile.c
+++ b/compile.c
@@ -7650,7 +7650,7 @@ ibf_load_object_string(const struct ibf_load *load, const struct ibf_object_head
     rb_enc_associate_index(str, encindex);
 
     if (header->internal) rb_obj_hide(str);
-    if (header->frozen)   rb_obj_freeze(str);
+    if (header->frozen)   str = rb_fstring(str);
 
     return str;
 }