Why does the subdomain finder stop because "label empty or too long"?

So I made this subdomain finder from a hacking course. It seems to work fine for a little bit but then gives a weird error. Could anyone help me?

Here is the code

        import requests

        url = "youtube.com"

        def request(url):
            try:
                return requests.get("http://" + url, timeout=2)
            except requests.exceptions.ConnectionError:
                pass
            except requests.exceptions.InvalidURL:
                print('INVALID:', url)

        with open("/home/kali/PycharmProjects/websitesub/subdomains-wodlist.txt", "r") as wordlist_file:
            for line in wordlist_file:
                word = line.strip()
                test_url = word + "." + url
                response = request(test_url)
                if response:
                    print("Discovered SUBDOM" + test_url) 

here is the output I am getting, as we can see it finds a few websites then stops