使用wx.openCard(OBJECT)需要code,但我wx.addCard(OBJECT)添加成功后返回的code是空字符串，这个怎么回事

wx.addCard({

cardList: [

{

cardId: card_id,

cardExt: '{"code":"","openid":"","timestamp":' + timestamp + ',"nonce_str":"' + nonceStr + '","signature":"' + dsignature + '"}'

}

],

success: function (res) {

console.log("卡券添加结果="+JSON.stringify(res)) // 卡券添加结果

}

})

appid是

wxcf6adf7d1484868d

cardId是

pq4MLuGhSFCmXPU8mNdwsmxDY2LY