title: Firebase FCM error: 'InvalidRegistration'
body: <p>I am currently trying to send a <code>PushNotification</code> to a <code>Device Group</code> using <code>FCM</code> with the help of <code>Firebase Cloud Functions</code> but once the notification is sent, it returns with code <code>200</code> but with failure :</p>
  
  <pre><code>SUCCESS response= {
  multicast_id: 8834986220110966000,
  success: 0,
  failure: 1,
  canonical_ids: 0,
  results: [ { error: 'InvalidRegistration' } ] 
  }
  </code></pre>
  
  <p>Here is the code I am using to send this notification... what am I missing?</p>
  
  <pre><code>const options = {
      method: 'POST',
      uri: 'https://fcm.googleapis.com/fcm/send',
      headers: {
         'Authorization': 'key=' + serverKey,
      },
      body: {
         to: groupId,
         data: {
          subject: message
         },
         notification: {
           title: title,
           body: body,
           badge: 1,
          },
         content_available: true
      },
      json: true
  };
  
  return rqstProm(options)
      .then((parsedBody) =&gt; {
          console.log('SUCCESS response=', parsedBody);
      })
      .catch((err) =&gt; {
          console.log('FAILED err=', err);
      });
  </code></pre>
  
  <p>Where JSON values <code>title</code>, <code>body</code>, <code>subject</code>, <code>message</code> are <code>String</code></p>