KeyError: 'SQLALCHEMY_TRACK_MODIFICATIONS'

#file database.py

from flask import Flask, g
from flask_sqlalchemy import SQLAlchemy
from sqlalchemy.ext.automap import automap_base
from sqlalchemy.orm import Session
from sqlalchemy import create_engine

db = None
Base = None

app = Flask(__name__)
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = 'False'

Base = automap_base()
engine = create_engine(app.config['SQLALCHEMY_DATABASE_URI'])
Base.prepare(engine, reflect=True)
db = SQLAlchemy(app)
db.init_app(app)

#file Estados
from flask_restful import Resource
import resources.database as database

class Estados(Resource):
    def get(self):
        estados = database.Base.classes.TBEstados
        est = database.db.session.query(estados).first()
        return est.nome