Firebase云消息传递日志错误

Question

我正在尝试设置推送通知，但是在Firebase日志中出现了一些错误，我似乎无法弄清原因。有人可以帮助修复我的代码。

我遇到的错误是：

未捕获的异常
提供给sendToDevice（）的注册令牌必须是非空字符串或非空数组

exports.observeFollowing = functions.database.ref('/follwoing/{uid}/{followingId}').onCreate((snapshot,context) => {

 var uid = context.params.uid;
 var followingId = context.params.trustingId;

 console.log('User: ' + uid + 'is trusting: ' + followingId);
 admin.database().ref('/users/' + followingId).once('value', snapshot => {

 var userWeAreFollowing = snapshot.val();

 admin.database().ref('/users/' + uid).once('value',snapshot => {

  var userDoingTheFollowing = snapshot.val();

  var payload = {
  notification: {
    title: "Someone new has followed you",
    body: "User XYZ has started trusting you...",
    sound: 'default'
  }
}

admin.messaging().sendToDevice(userWeAreFollowing.fcmToken, payload)
      .then((response) => {
        console.log('Successfully sent message:', response);
        return response
      })
      .catch((error) => {
        console.log('Error sending message:', error);
      });
})
})
})