diff --git a/hash.c b/hash.c
index 83d1799..0e492cf 100644
--- a/hash.c
+++ b/hash.c
@@ -1248,7 +1248,10 @@ static int
 hash_aset_str(st_data_t *key, st_data_t *val, struct update_arg *arg, int existing)
 {
     if (!existing) {
-	*key = rb_str_new_frozen((VALUE)*key);
+	VALUE str = (VALUE)*key;
+
+	if (!OBJ_FROZEN(str))
+	    *key = rb_fstring(str);
     }
     return hash_aset(key, val, arg, existing);
 }