diff --git a/ChangeLog b/ChangeLog
index 0e05b9e058..25382d68bf 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Wed Oct  9 17:40:16 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_fstring): because of lazy sweep, str may be unmaked
+	  already and swept at next time, so mark it for the time being.
+	  [ruby-core:57756]
+
 Wed Oct  9 13:53:14 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* compar.c (cmp_eq): fail if recursion.  [ruby-core:57736] [Bug #9003]
diff --git a/string.c b/string.c
index 7bf4006fdf..6512fe4092 100644
--- a/string.c
+++ b/string.c
@@ -146,6 +146,9 @@ rb_fstring(VALUE str)
     st_data_t fstr;
     if (st_lookup(frozen_strings, (st_data_t)str, &fstr)) {
 	str = (VALUE)fstr;
+	/* because of lazy sweep, str may be unmaked already and swept
+	 * at next time */
+	rb_gc_mark(str);
     }
     else {
 	str = rb_str_new_frozen(str);