TypeError: Request url must be str or unicode, got list:

import scrapy
from ..items import HamrobazarItem

class CarsSpider(scrapy.Spider):
name = 'cars'
start_urls = ['https://hamrobazaar.com/c48-automobiles-cars']

def parse(self, response):
    items= HamrobazarItem()
    img_urls=list()
    img_urls.append(response.css('center img::attr(src)').extract())
    items['image_urls']=img_urls

    yield items

I am trying to extract and save the image but everytime I run spider I am getting TypeError: Request url must be str or unicode, got list: , I have defined following functions in items.py

import scrapy
class HamrobazarItem(scrapy.Item):

    images=scrapy.Field()
    image_urls=scrapy.Field()
    pass