Scrapy - Selenium TypeError: 'NoneType' object is not iterable

I get the following error:

start_requests = iter(self.spider.start_requests())
   TypeError: 'NoneType' object is not iterable

when I run:

class CramoSpider(Spider):
    name = 'cramo'
    allowed_domains = ['www.cramo.ee']

    def start_requests(self):
        self.driver = webdriver.Chrome('/Users/Laptop/chromedriver')
        self.driver.get('https://www.cramo.ee/et/rentimine/hinnakiri')

    def parse_book(self, response):
        pass

What am I doing wrong here?

Thank you