canvas生成图片返回create bitmap failed什么意思？

wx.canvasToTempFilePath({
    canvasId: 'firstCanvas',
    success: function (res) {
        console.log(res.tempFilePath);
        that.setData({
            shareImgUrl:res.tempFilePath,
            isShow:true
        });
    },
    fail: function (res) {
        console.log(res);
    },
    complete: function (res) {
        console.log(res);
        wx.hideLoading();
    }
});