Paypal webhook resource is null

Question

I have enabled the INVOICING.INVOICE.PAID webhook in my application under developement. After trying to pay an invoice in the sandbox environment (using the buyer test user) the webhook notification is successfully delivered, however the message does not containt the actual invoice data.

As you can see in the JSON below, the resource.invoice property is null.

How can I get the actual data and find the id of the invoice that has been paid?

The webhook notification I receive:

{
"id": "WH-6L999276D66321518-9G54023288302824Y",
"create_time": "2017-01-18T22:58:08Z",
"resource_type": "invoices",
"event_type": "INVOICING.INVOICE.PAID",
"summary": "An invoice has been paid",
"resource": {
    "invoice": null
},
"status": "PENDING",
"transmissions": [
    {
        "webhook_url": "https://6fbe15c2.ngrok.io/scc/webhooks",
        "response_headers": {
            "Content-Length": "34",
            "SERVER_INFO": "",
            "Content-Type": "text/plain",
            "HTTP/1.1 404 Not Found": ""
        },
        "transmission_id": "945c0220-ddd1-11e6-bc94-77339302725b",
        "status": "PENDING",
        "timestamp": "2017-01-18T22:58:08Z"
    }
],
"links": [
    {
        "href": "https://api.sandbox.paypal.com/v1/notifications/webhooks-events/WH-6L999276D66321518-9G54023288302824Y",
        "rel": "self",
        "method": "GET",
        "encType": "application/json"
    },
    {
        "href": "https://api.sandbox.paypal.com/v1/notifications/webhooks-events/WH-6L999276D66321518-9G54023288302824Y/resend",
        "rel": "resend",
        "method": "POST",
        "encType": "application/json"
    }
],
"event_version": "1.0"
}