argument of type 'NoneType' is not iterable POST запросы

@app.route('/transactions/new', methods=['POST'])
def GVO_new_transaction():
    values = request.get_json()

    required = ['sender', 'recipient', 'amount']
    if not all(k in values for k in required):
        return 'Missing values', 400

    index = blockchain.GVO_new_transaction(values['sender'], values['recipient'], values['amount'])
    response = {'message': f'Transaction will be added to Block {index}'}
    return jsonify(response), 201

Вот скрин ошибки: