diff --git a/gc.c b/gc.c
index 6712dd9e..38be3150 100644
--- a/gc.c
+++ b/gc.c
@@ -480,6 +480,9 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
     for (it = xh_begin(&data->storage); it != NULL; it = xh_next(it)) {
       gc_mark(pic, xh_val(it, pic_value));
     }
+    if (data->type->mark) {
+      data->type->mark(pic, data->data, gc_mark);
+    }
     break;
   }
   case PIC_TT_DICT: {
diff --git a/include/picrin/data.h b/include/picrin/data.h
index 0e59095d..79b633a5 100644
--- a/include/picrin/data.h
+++ b/include/picrin/data.h
@@ -12,6 +12,7 @@ extern "C" {
 typedef struct {
   const char *type_name;
   void (*dtor)(pic_state *, void *);
+  void (*mark)(pic_state *, void *, void (*)(pic_state *, pic_value));
 } pic_data_type;
 
 struct pic_data {