1 files changed, 3 insertions, 0 deletions
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);