How can I fix 'greenlet_spawn has not been called' error when using async_sessionmaker with FastAPI and SQLAlchemy?

FastAPI + SQLAlchemy + Alembic + async_sessionmaker not working

When "alembic revision --autogenerate' I get an error 'sqlalchemy.exc.MissingGreenlet: greenlet_spawn has not been called; can't call await_only() here. Was IO attempted in an unexpected place?'

why doesn't this work?

db.py:

from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine, async_sessionmaker
from sqlalchemy.ext.declarative import declarative_base

from core.config import config

Base = declarative_base()
engine = create_async_engine(config.DB_URL)
async_session = async_sessionmaker(engine, expire_on_commit=False, class_=AsyncSession)

model - user.py:

from sqlalchemy import String
from sqlalchemy.orm import Mapped, mapped_column

from core.db.session import Base


class User(Base):
    __tablename__ = 'users'

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str] = mapped_column(String)

env.py

from core.config import config as app_config
from logging.config import fileConfig

from sqlalchemy import engine_from_config
from sqlalchemy import pool

from alembic import context


config = context.config
context.config.set_main_option('sqlalchemy.url', app_config.DB_URL)

if config.config_file_name is not None:
    fileConfig(config.config_file_name)

from app.user.models.user import *
from core.db.session import Base

target_metadata = Base.metadata
...

alembic init -t async migrations resolve this problem