Error with the "start" command in the telegram bot?

Question

Python

Start command script:

@dp.message_handler(commands=['start'])
async def start_command(message: types.Message):
    with open('pol.txt') as file:
       contents = file.read()
       search_word = message.from_user.username
    if search_word in contents:
        pass
    else:
        file = open("pol.txt", "a")
<b>        file.write(message.from_user.username + ';' + message.from_user.id + '\n')</b>
        file.close()
    await bot.send_message(chat_id=message.chat.id, text=' Привет, я помогу тебе скачать видео с TikTok. \n/help - инструкция как скачать видео.\n \n Поддержать проект - qiwi.com/n/XACER\n Магазин - @eze_pokupka3_bot')

Error on start command:

Task exception was never retrieved
future: <Task finished name='Task-9' coro=<Dispatcher._process_polling_updates() done, defined at /usr/local/lib/python3.9/dist-packages/aiogram/dispatcher/dispatcher.py:409> exception=TypeError('can only concatenate str (not "int") to str')>
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/dist-packages/aiogram/dispatcher/dispatcher.py", line 417, in _process_polling_updates
    for responses in itertools.chain.from_iterable(await self.process_updates(updates, fast)):
  File "/usr/local/lib/python3.9/dist-packages/aiogram/dispatcher/dispatcher.py", line 238, in process_updates
    return await asyncio.gather(*tasks)
  File "/usr/local/lib/python3.9/dist-packages/aiogram/dispatcher/handler.py", line 116, in notify
    response = await handler_obj.handler(*args, **partial_data)
  File "/usr/local/lib/python3.9/dist-packages/aiogram/dispatcher/dispatcher.py", line 259, in process_update
    return await self.message_handlers.notify(update.message)
  File "/usr/local/lib/python3.9/dist-packages/aiogram/dispatcher/handler.py", line 116, in notify
    response = await handler_obj.handler(*args, **partial_data)
  File "/root/TT2/main.py", line 38, in start_command
    file.write(message.from_user.username + ';' + message.from_user.id + '\n')
TypeError: can only concatenate str (not "int") to str

What am I doing wrong in the code?