Firebase Firestore client cannot be deployed

I have an Firebase Cloud Functions codebase that uses Firestore database. Everything below works when I use it in local emulator using firebase emulators:start but when I need to deploy it to Firebase I got below error:

Error: User code failed to load. Cannot determine backend specification

main.py

import json
from firebase_functions import https_fn
from firebase_admin import initialize_app, firestore
import flask
from enum import Enum
from flask import g
from endpoints.moon_phase import moon_phase_bp

# Initialize Firebase app and Firestore
initialize_app()
db = firestore.client()
app = flask.Flask(__name__)


# Set up a before_request function to make db available in blueprints
@app.before_request
def before_request():
    # g.db = db
    print("before_request")

app.register_blueprint(moon_phase_bp) //Doesn't even use db, but in the future it will.


# Firebase Function to handle requests
@https_fn.on_request()
def astro(req: https_fn.Request) -> https_fn.Response:
    with app.request_context(req.environ):
        return app.full_dispatch_request()

If I update the db initialisation to db = firestore.client it deploys, but obviously, it’s a function reference, so I cannot use Firestore db in my endpoints. This also means it’s not related to my Firebase credentials or project setup.

What might be the issue here?