/send_email is 404

Why does this fail?

URL for valid invoice https://api.freeagent.com/v2/invoices/65884101/send_email

Test bodies…

{
  "invoice": {
    "email": {
      "use_template": true
    }
  }
}
{
   "invoice":{
      "email":{
         "to":"recipient@example.com",
         "from":"sender@example.com",
         "subject":"Subject",
         "body":"Body"
      }
   }
}

POST with Content-Type application/JSON results in [400] Format not recognised. Set accept http header to application/json or application/xml.

POST with accepts application/JSON results in [404] Resource not found.

The invoice, however, is confirmed as existing. It contains a test item.