diff --git a/ChangeLog b/ChangeLog
index 113ed9c892..93f0fe2853 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Mar 18 12:07:36 2015  Koichi Sasada  <ko1@atdot.net>
+
+	* string.c: add a comment about RSTRING_FSTR.
+
 Wed Mar 18 12:01:53 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* hash.c (rb_any_hash): use same hash values with Float#hash so
diff --git a/string.c b/string.c
index 68fcfc2852..f770118660 100644
--- a/string.c
+++ b/string.c
@@ -60,6 +60,7 @@ VALUE rb_cSymbol;
  * 7:     STR_TMPLOCK
  * 8-9:   ENC_CODERANGE (2 bits)
  * 10-16: ENCODING (7 bits == 128)
+ * 17:    RSTRING_FSTR
  * 18:    STR_NOFREE
  * 19:    STR_FAKESTR
  */