MongooseError: The uri parameter to openUri() must be a string

Question

I have the following situation in a NextJS app on Heroku.

In the root directory there are the app folder and a lib folder. The app folder contains a subfolder (components). The lib folder contains a file (mongoDB.tsx), which contents is:

import mongoose from "mongoose"

const uri = process.env.MDB_URI_AUDIO!,
            connectMDB = async () => mongoose.connect(uri)

export default connectMDB

At this point I can run successfully this command:

% npm run build

From now on I want to add a new route to the app, and this is where troubles are going to show up.

In the app folder I add a subfolder (Channels_Pool) for the new route. And Channels_Pool contains a file (page.tsx), which contents is:

import ChannelList from '../components/ChannelList'

export default function NxtJSRoute() {

  return (
    <>
    <ChannelList />
    </>
  )
}

And this is the contents of ChannelList (inside components):

import channelsPool from "@/lib/channels";

export default async function ChannelList() {
  const theResult = await channelsPool() // This leads to an error !!

  return <div>
  {
    <div>Hello-ChannelList</div>
    // More useful code is to come here later, making use of theResult.
  }
  </div>
} /* End of ChannelList */

Running now this command:

% npm run build

leads to a failure with this error:

MongooseError: The `uri` parameter to `openUri()` must be a string, got "undefined". Make sure the first parameter to `mongoose.connect()` or `mongoose.createConnection()` is a string.

I have been able to check that this line is causing the problem:

const theResult = await channelsPool()

Can someone see what I am doing wrong and what the solution is ?