SQLalchemy NOT NULL constraint failed on primary key

I am making an inventory database using SQLalchemy, but have some struggles with it. My code looks like this:

@cross_origin
@app.route('/api/add_item_loan', methods=['POST'])
def add_item_loan():
    data = request.get_json()
    item = Item.query.filter_by(description=data['description']).first()
    iid = item.iid
    itemLoan = ItemUse(iid=iid, quantity=int(data['quantity']))
    db.session.add(itemLoan)
    db.session.commit()
    return {'message': 'Loan added succesfully'}, 201

and my models.py looks like this:

class Item(db.Model):
    __tablename__ = 'items'
    
    iid = db.Column(db.Integer, primary_key=True)
    description = db.Column(db.Text, nullable=False)
    quantity = db.Column(db.Integer, nullable=False)

class ItemUse(db.Model):
    __tablename__ = 'item_usage'
    
    iuid = db.Column(db.Integer, primary_key=True)
    iid = db.Column(db.Integer, db.ForeignKey('items.iid'), nullable=False)
    quantity = db.Column(db.Integer, nullable=False)
    start_date = db.Column(db.DateTime, default=date.today())
    end_date = db.Column(db.DateTime, nullable=True)

and when I try to add an item, I get this error:

sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) NOT NULL constraint failed: item_usage.iuid
[SQL: INSERT INTO item_usage (iid, quantity, start_date, end_date) VALUES (?, ?, ?, ?)]
[parameters: (1, 2, '2025-10-17 00:00:00.000000', None)]
(Background on this error at: https://sqlalche.me/e/20/gkpj)

I have searched a little here, and tried quite some things, but nothing seems to work.

The database is sqlite, and I use flask migrate.

Edit: I had to delete my migrations folder and my database, and now everything is working, solved