Why can't I reach the property?

Question

Vue.js

<div>{{my_data['2018-07-11']}} </div>
Output:
{ "may": 4, "june": 8, "april": 5 }
This code:
<div>{{my_data['2018-07-11']['may']}} </div>
Output 4. But on the console I get an error:

Error in render: "TypeError: Cannot read property 'may' of undefined"

The application code itself:

var app = new Vue({
  el: '#app',
  data: {
    my_data: {},
    mess: 'hello world'
  },

     created: async function(){
    	url = 'http://dlang.ru/test'
    const response = await fetch(url);
    this.my_data = await response.json();

    } 	
})

Answer 1

Because at the time of rendering the template, the data has not yet arrived and the object with the data is empty.