Invalid character in header

from asyncio import run
from aiohttp import ClientSession
async def main():
    r=await s.get('')

It was some time ago, but I have it in my notes that it may be for aiohttp/5269.
This gives exception:
aiohttp.http_exceptions.BadHttpMessage: 400, message=‘invalid character in header’

aiohttp: 3.7.4.post0
python: 3.9.4