Vue data() is unavailable in events when using @microsoft/fetch-event-source for Server Sent Events

Question

I'm trying to use fetchEventSource (from @microsoft/fetch-event-source) to process Server Sent Events for streaming data to the client. It works, in that the data is returned and I can log it to the console, but when I try to update my vue data in the onmessage event, I get the error:

TypeError: Cannot set properties of undefined (setting 'messages')

Any thoughts on the best way to get this working?

Basic test template:

<template>
  <div>
    <p>
      <label>Message</label><br />
      <textarea id="messages" v-model="messages"></textarea><br />
      <button @click="getMessage">GET</button>
    </p>
  </div>
</template>

and the script:

import { fetchEventSource } from "@microsoft/fetch-event-source";

export default {
  data() {
    return {
      messages: "",
    };
  },
  mounted() {
    this.messages = "initialized"; // this works
  },
  methods: {
    getMessage() {
      fetchEventSource("url-here", {
        method: "POST",
        headers: {
          "Content-Type": "application/json",
        },
        onmessage(msg) {
          console.log("message", msg.data); // this works - data is here!
          this.messages = msg.data; // this returns the error
        },
      });
    },
  },
};