title: Error: "data must only contain string values" firebase cloud messaging
body: <p>I'm trying to send some data from my node.js server to an android client via FCM(Firebase Cloud Messaging). I get the following error: "data must only contain string values" when sending it. My data contains 2 JSONObjects. Do I have to convert them to strings or what is the way to go here? Thanks.</p>
  
  <pre><code>var message = {
      notification:{
          "title": "Alert!",
          "body": position[0] + " has left Area: " + activeGeofences[i][1].name
        },
        data:{
          Geofence: activeGeofences[i][1],
          Position: position[1]
        },
      token: activeGeofences[i][0]
    };
  </code></pre>