Cannot json_encode() a PHP object (RESTful Exception Error object)

Question

I am trying to convert a RESTful Exception Error object to a string to save it and send an email.

I get the error json_encode(): type is unsupported, encoded as null. From Googling around, it seems like json_encode is not supposed to have problems, but it doesn't work for resource variables.

This is what the object looks like:

RESTful\Exceptions\HTTPError Object
(
    [response] => Httpful\Response Object
        (
            [body] => stdClass Object
                (
                    [status] => Bad Request
                    [category_code] => request
                    [additional] => 
                    [status_code] => 400
                    [extras] => stdClass Object
                        (
                            [card_uri] => "None" must be a string URI
                        )

                    [category_type] => request
                    [_uris] => stdClass Object
                        (
                        )

                    [request_id] => XXX
                    [description] => Invalid field [card_uri] - "None" must be a string URI Your request id is XXX.
                )

            [raw_body] ⇒ {
  "status": "Bad Request",
  "category_code": "request",
  "additional": null,
  "status_code": 400,
  "extras": {
    "card_uri": "\"None\" must be a string URI"
  },
  "category_type": "request",
  "_uris": {},
  "request_id": "XXX",
  "description": "Invalid field [card_uri] - \"None\" must be a string URI Your request id is XXX."
}