mrb_mod_included_modules(mrb_state *mrb, mrb_value self)
{
  mrb_value result;
  struct RClass *c = mrb_class_ptr(self);
  struct RClass *origin = c;

  MRB_CLASS_ORIGIN(origin);
  result = mrb_ary_new(mrb);
  while (c) {
    if (c != origin && c->tt == MRB_TT_ICLASS) {
      if (c->c->tt == MRB_TT_MODULE) {
        mrb_ary_push(mrb, result, mrb_obj_value(c->c));
      }
    }
    c = c->super;
  }

  return result;
}

The product dereferences a pointer that it expects to be valid but is NULL.