HTML response for forbidden API call

When making an API call that results in an HTTP 403 response, HTML is returned:
```

<html>
<head><title>403 Forbidden</title></head>
<body>
<center><h1>403 Forbidden</h1></center>
</body>
</html>
```

Shouldn't this be JSON?

This is sporadically triggered against: https://sync.standardnotes.org (unknown reason) so may be AWS infrastructure returning, rather than server code.