TypeError: Converting circular structure to JSON -- Next.js

I'm writing an API in Next.js to insert data into a MySQL database. The error I'm experiencing relates to a circular reference, but I'm having trouble finding it. Please be aware that I am also using Axios to write this API.

I'm running into the following error:

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Query'
    |     property '_timer' -> object with constructor 'Timer'
    --- property '_object' closes the circle

Here is my API:

import type { NextApiRequest, NextApiResponse } from "next";
import * as pool from "../../../src/utils/dbConnection";
import console from "console";

export default async (req: NextApiRequest, res: NextApiResponse) => {
  const { email, password, firstname, lastname } = req.body as any;
  let conn;
  try {
    conn = await pool.getConnection();
    const rows = await conn.query(
      `INSERT INTO Users(email, password, firstname, lastname) VALUES(?, ?, ?, ?)`,
      [email, password, firstname, lastname]
    );

    res.status(200).json(JSON.stringify(rows));
  } catch (err) {
    console.log(err);
  } finally {
    if (conn) conn.end(); // close connection
  }
};