Pythonでinvalid argument: 'url' must be a stringとエラーがでる

前提・実現したいこと

エラーを解消したい。

発生している問題・エラーメッセージ

プログラムを実行した所、下記のエラーが出てしまう。

Traceback (most recent call last):
  File "test.py", line 73, in <module>
    browser.get(btn)
  File "/Users/私のPCの名前/.pyenv/versions/3.7.5/lib/python3.7/site-packages/selenium/webdriver/remote/webdriver.py", line 333, in get
    self.execute(Command.GET, {'url': url})
  File "/Users/私のPCの名前/.pyenv/versions/3.7.5/lib/python3.7/site-packages/selenium/webdriver/remote/webdriver.py", line 321, in execute
    self.error_handler.check_response(response)
  File "/Users/私のPCの名前/.pyenv/versions/3.7.5/lib/python3.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 242, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.InvalidArgumentException: Message: invalid argument: 'url' must be a string
  (Session info: chrome=85.0.4183.59)

該当のソースコード

import sys
import os
from selenium import webdriver
import pandas
import time
import chromedriver_binary

browser = webdriver.Chrome()

#1

args = sys.argv
df = pandas.read_csv('default.csv', index_col=0)

#3

browser.get("https://coconala.com/categories/8")

#4

page = 1

#5

while True: #continue until getting the last page

    #5-1

    if len(browser.find_elements_by_css_selector("h3")) > 0:
        print("######################page: {} ########################".format(page))
        print("Starting to get posts...")

        #5-1-2

        time.sleep(5)

        posts = browser.find_elements_by_css_selector(".listContentBox") #ページ内のタイトル複数
        print (len(posts))

        #5-1-3

        for post in posts:

            try:

                title = post.find_element_by_css_selector("a.js-service-view-tracker").text
                print(title)

                detail = post.find_element_by_css_selector("h3").text
                print(detail)

                #5-1-3-1

                price = post.find_element_by_css_selector("strong.red").text
                print(price)
                #5-1-3-2

                liked = post.find_element_by_css_selector("span.overlay").text
                print(liked)

                url = post.find_element_by_css_selector("a.js-service-view-tracker").get_attribute("href")
                se = pandas.Series([title,detail, price, liked,url],['title', 'detail','price','liked','url'])
                df = df.append(se, ignore_index=True)
            except:
                print("Error:Advertisement appeared.Skipping...")

        #5-1-4

        page+=1

        btn = browser.find_element_by_css_selector("h3").get_attribute("href")
        print("next url:{}".format(btn))
        browser.get(btn)
        print("Moving to next page......")

    #5-2

    else:
        print("no pager exist anymore")
        break
#6
print("Finished Scraping. Writing CSV.......")
df.to_csv("output.csv")
print("DONE")