获取小程序二维码返回的二进制转成base64前端为什么显示不出来

<image :src="qrcodeUrl" mode="aspectFill" style="width: 100%;height: 100%;"></image>

js代码

wx.request({    

    url: IP + 'feed/member/invite/bdwxqrcode',

    method: 'GET',

header: {

"Authorization": `Bearer ${uni.getStorageSync("token")}` || ''

},

responseType: 'arraybuffer',

    success: function (res) {

let base64Img = wx.arrayBufferToBase64(res.data);

_this.qrcodeUrl = 'data:image/png;base64,' + base64Img

    }

})