diff --git a/ChangeLog b/ChangeLog
index 318fd1b0d7..58db8e9d9d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Thu Oct 17 18:45:41 2013  Koichi Sasada  <ko1@atdot.net>
+
+	* string.c (STR_HEAP_SIZE): includes TERM_LEN(str).
+
+	* string.c (rb_str_memsize): use STR_HEAP_SIZE().
+
 Thu Oct 17 17:43:00 2013  Shugo Maeda  <shugo@ruby-lang.org>
 
 	* vm_insnhelper.c (vm_call_method): set ci->me to 0 when the
diff --git a/string.c b/string.c
index b81f839eb4..3c689bda40 100644
--- a/string.c
+++ b/string.c
@@ -127,7 +127,7 @@ VALUE rb_cSymbol;
 } while (0)
 
 #define STR_HEAP_PTR(str)  (RSTRING(str)->as.heap.ptr)
-#define STR_HEAP_SIZE(str) (RSTRING(str)->as.heap.aux.capa)
+#define STR_HEAP_SIZE(str) (RSTRING(str)->as.heap.aux.capa + TERM_LEN(str))
 
 #define is_ascii_string(str) (rb_enc_str_coderange(str) == ENC_CODERANGE_7BIT)
 #define is_broken_string(str) (rb_enc_str_coderange(str) == ENC_CODERANGE_BROKEN)
@@ -891,7 +891,7 @@ RUBY_FUNC_EXPORTED size_t
 rb_str_memsize(VALUE str)
 {
     if (!STR_EMBED_P(str) && !STR_SHARED_P(str)) {
-	return RSTRING(str)->as.heap.aux.capa;
+	return STR_HEAP_SIZE(str);
     }
     else {
 	return 0;