Firebase FCM error: 'InvalidRegistration'

I am currently trying to send a PushNotification to a Device Group using FCM with the help of Firebase Cloud Functions but once the notification is sent, it returns with code 200 but with failure :

SUCCESS response= {
multicast_id: 8834986220110966000,
success: 0,
failure: 1,
canonical_ids: 0,
results: [ { error: 'InvalidRegistration' } ] 
}

Here is the code I am using to send this notification... what am I missing?

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) => {
        console.log('SUCCESS response=', parsedBody);
    })
    .catch((err) => {
        console.log('FAILED err=', err);
    });

Where JSON values title, body, subject, message are String

via John