invalid json response body at http://localhost:3000/api/user/[object%20Object] reason: Unexpected token < in JSON at position 0 - json

Question

I'm using this example to create a simple authentication with Nextjs https://github.com/vvo/next-iron-session/tree/master/examples/next.js
but instead of fetching the user JSON object from Github (as the example does) im trying to do it from my mongodb database where i have some users.
I did this on my login.js file:
import fetchJson from "../../lib/fetchJson";
import withSession from "../../lib/session";
import { withIronSession } from "next-iron-session";
import { connectToDatabase } from "../../util/mongodb";
export default withSession(async (req, res) => {
const { db } = await connectToDatabase();
const { username } = await req.body;
const foundUser = await db.collection("users").findOne({"userName": username});
console.log(foundUser) // <--- this returns the user object on console just fine
const url = `http://localhost:3000/api/user/${foundUser}`;
try {
const { userName, email } = await fetchJson(url);
const user = { isLoggedIn: true, userName, email }
req.session.set("user", user);
await req.session.save();
res.json(user);
} catch (error) {
const { response: fetchResponse } = error;
res.status(fetchResponse?.status || 500).json(error.data);
}
});
And i have this code on my /api/user.js file:
import withSession from "../../lib/session";
import { connectToDatabase } from "../../util/mongodb";
export default withSession(async (req, res) => {
const user = req.session.get("user");
if (user) {
const { db } = await connectToDatabase();
const foundUser = await db.collection("users").findOne({"userName": user.userName, "email": user.email});
console.log("useri pi te user.js " + foundUser)
// in a real world application you might read the user id from the session and then do a database request
// to get more information on the user if needed
res.json({
isLoggedIn: true,
...user,
});
} else {
res.json({
isLoggedIn: false,
});
}
});
But i get "invalid json response body at http://localhost:3000/api/user/[object%20Object] reason: Unexpected token < in JSON at position 0" error even though i get the user object printed in the console just fine.
Any help would be appreciated!