NOT NULL constraint failed in SQLachemy Flask webapp

I have recently started working with database. I am trying to set up a database with flask and SQLalchemy. The models.py and database.py are shown as below:

#models.py

mport sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Expense(Base):
    __tablename__ = 'expense'
    id = sa.Column(sa.BigInteger, primary_key=True)
    username = sa.Column(
                        sa.String,
                        info={'label': 'Name'},
                        nullable=False
    )
    def __init__(self, username):
        self.username = username

    def __repr__(self):
        return '<User %r>' % (self.username)

and

#database.py

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from models import Base, Expense


def init_db():
    engine = create_engine('sqlite:///test.db')
    Base.metadata.create_all(engine)
    Base.metadata.bind = engine
    DBSession = sessionmaker(bind=engine)
    session = DBSession()

    new_entry = Expense(username="noobie")
    session.add(new_entry)
    session.commit()

init_db()

I am running these on python3, and when I run database.py, I get

sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed: expense.id [SQL: 'INSERT INTO expense (username) VALUES (?)'] [parameters: ('noobie',)]

One response here suggests that using sqlite_autoincrement=True will solve the problem, but it didn't work on mine. I think the problem occurs somewhere between SQLite3 and sqlalchemy when assigning the primary key... I don't know neither well. Please help this poor noobie, thanks!