Why is the length of the object received from storage not determined in Vuex?

Question

Vue.js

I'm using Vuex storage to filter news by author. Here are the main elements:

Vue.use(Vuex)
  const store = new Vuex.Store({
    state: {
        news: null,
      author_id:'all'
      },
    getters: {
      NEWS: state => {
        return state.news;
      },
      AUTHOR: state => {
        return state.author_id;
      }
    },
    mutations: {
      SET_NEWS: (state, payload) => {
          state.news = payload;
      },
      SET_AUTHOR: (state, payload) => {
          state.author_id = payload;
      }
    },
    actions: {
      GET_NEWS: async (context, author_id='all') => {
        let data = await axios.get('http://news/author_'+author_id).then(response => {return response.data});
              context.commit('SET_NEWS', data);
      },
      GET_AUTHOR: async (context,author_id) => {
              context.commit('SET_AUTHOR', author_id);
      }
    }
  });
  const app = new Vue({
    el: '#content',
    store,
    data: {
        selected_author: 'all'
    },
    mounted() {
      this.$store.dispatch('GET_NEWS','all');
    },
    methods:{
      async onGetNews () {
      			await this.$store.dispatch('GET_NEWS',this.selected_author);
      			console.log(this.$store.getters.NEWS.length);
      		}
    }
  });

When calling the onGetNews method in the console, it says: Error in render: "RangeError: Invalid array length", and the array length is undefined.
If called in onGetNews(): then it displays the object:
console.log(this.$store.getters.NEWS)

{__ob__: Observer}
12: (...)
13: (...)
__ob__: Observer {value: {…}, dep: Dep, vmCount: 0}
get 12: ƒ reactiveGetter()set 12: ƒ reactiveSetter(newVal)
get 13: ƒ reactiveGetter()set 13: ƒ reactiveSetter(newVal)
__proto__: Object

Why does not determine the length of this object? How to determine?