Accessing VUE JS's data from Axios

I have a Vue JS (Vuetify) App that makes an ajax request that I would like to populate a div's content with the response, however I am having difficulties accessing the instance's data. All examples I have seen use this to point to the data object, but when I do I get this error

Unable to set property 'message' of undefined or null reference

The app is quite simple:

main.js:

import Vue from 'vue'
import App from './App.vue'
import Vuetify from 'vuetify'


Vue.use(Vuetify)

new Vue({
  el: '#app',
  render: h => h(App)
})

App.vue

export default {
  data () {
    return {
    ....
    message: '',
    order: {},
    ...
  },
  methods: {
    send: function() {
      axios.post(this.api+"orders",this.order).then(function(response) {
        this.message = "Your payment was successful";
        ...
      }
   }
 }

this.order is accessible without a problem with Axios' post method however the anonymous function that handles the promise returned seems to have a problem accessing this.message, in contrary to the examples I have seen.

What is it that I am doing differently here?