title: FCM iOS: Invalid 'Sound' in payload
body: <p>I am attempting to do push notifications to my iOS app using FCM but am having problems with the sound. I followed this <a href="https://stackoverflow.com/questions/39335363/fcm-notification-in-ios-doesnt-play-sound-when-received">post</a> and implemented the 'sound' key-value pair accordingly but firebase throw this error. Could anyone advice pls?</p>
  
  <blockquote>
    <p>Error sending message: { Error: Invalid JSON payload received. Unknown
    name "sound" at 'message.notification': Cannot find field.</p>
  </blockquote>
  
  <p>My JSON implementation on cloud functions as follows:</p>
  
  <pre><code>...
  return admin.database().ref('/fcmToken/' + userUid).once('value', snapshot =&gt; {
          const values = snapshot.val()
          const fcmToken = values.fcmToken
  
          var message = {
              notification: {
                  body: 'New message(s)',
                  sound: 'default'
              },
  
                token: fcmToken
              };
  
          return admin.messaging().send(message)
            .then((response) =&gt; {
              return console.log('Successfully sent message:', response);
            })
            .catch((error) =&gt; {
              return console.log('Error sending message:', error);
            });
      })
  </code></pre>