React Native Push Notifications, FCM says token is invalid on iOS only (Android works)

We have a React Native App which is trying to send push notifications using the Firebase Cloud Messaging Service. We are testing this using Insomina.

To do this, we are using a package called “react-native-push-notification” by zo0r.We’ve been able to obtain an iOS apn token from a real life apple device. We are then able to successfully send a POST request to this url:

https://iid.googleapis.com/iid/v1:batchImport 

to convert the apns token into a registration token for Google’s firebase cloud messaging platform.

However, when we try and send a message using the converted token, by sending a POST request to this endpoint:

https://fcm.googleapis.com/fcm/send

using this JSON payload:

{"direct_book_ok": true,"to": "fTiQ5mg266o:APA91bGQN1s9sYJh9U_B6h1vuwAjpQVrPIJFnCWWsiv4PgKTCOCqLiKIdAD6ls48f-dBbnV3tOdc6NvTScXSRJYrXNHz_5QonSFraEau5jKSJPcw8HYeOb2YnutC39TlaIlKBnD3FpSx","notification": {"title": "Breaking News","body": "New Story available."              },"priority": "high"}

AND these headers:

Authorization: key=<OUR_SECRET_KEY> and Content-Type: application/json

The Google API responds with the following error, indicating that the notification has failed because of an “invalid argument”.

{"multicast_id": 3503657462083604439,"success": 0,"failure": 1,"canonical_ids": 0,"results": [   {"error": "InvalidParameters: RpcError:INVALID_ARGUMENT"   } ]}

How would we remedy this? We’ve tried both sandbox=true and sandbox false.