TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received an instance of Object

const SMTPServer = require('smtp-server').SMTPServer;

const SpamScanner = require('spamscanner');
const scanner = new SpamScanner();

var streamContent = {};
var sessions = {};

const server = new SMTPServer({
    onData(stream, session, callback) {
        sessions[session.id] = {...session}
        stream.on('data', function(d) {
            if (typeof streamContent[session.id] == "undefined") {
                streamContent[session.id] = {}
                streamContent[session.id].msg = d.toString('utf8');
            } else
                streamContent[session.id].msg += d.toString('utf8');
        })
        stream.on('end', function() {
            callback(false)
            process(session.id)
        });
    },
    authOptional: true
});

function process(session) {
    console.log(typeof streamContent[session].msg)
    console.log(streamContent[session].msg)
    scanner.scan(streamContent[session].msg, (err, scan) => {
      if (err) 
        console.error(err);
    });
}

server.listen(20024, function(){
    console.log('running')
});

running
string
Received: by mx01.xxxx.com (Postfix, from userid 0)
        id 718F041789; Fri, 15 Nov 2024 17:39:27 +0100 (CET)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=xxxx.com;
        s=mailer; t=;
        bh=g3zLYH4xKxcPrHOD18z9YfpQcnk/GaJedfustWU5uGs=;
        h=Subject:To:Date:From:From;
        b=fwUBTovFAXu1B54QJ1B8M7CcarR0hFprFb6ErjiPmyNmqjV1R9IS0ss29JotVXeJW
         b3CQLh2B/wD7AbvbUNnP8rcvClCLAs97y+35lqw6+ejNsf9qPpSSQ1y3LCM/ionM3H
         Taki7SCMJVVirFKzx+vOGEcaABS1Zvt0jUNVCaeieKAVzD0yKUREcqxnQc0/zkQrCc
         BzV7M4Hg+TW9B4/yiyS+WcELQP9+CNhZF5CtqF92FUCQdepv53ewsneFH6YnuToGfM
         /oDV3MKyJizcP9NO+C1khNC87cM3ZBk/udvpsuNmYbcdtuXNXNdkP7rhdm5ZnodwV8
         VXIRC4WPrwJPA==
Subject: test
To: <philipp@xxxx.com>
X-Mailer: mail (GNU Mailutils 3.7)
Message-Id: <20241115163927.718F041789@mx01.xxxx.com>
Date: Fri, 15 Nov 2024 17:39:27 +0100 (CET)
From: root <root@xxxx.com>

test

node:internal/streams/writable:480
      throw new ERR_INVALID_ARG_TYPE(
      ^

TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received an instance of Object
    at _write (node:internal/streams/writable:480:13)
    at Writable.end (node:internal/streams/writable:821:17)
    at Immediate.<anonymous> (/node/postfix/node_modules/nodemailer/lib/mime-node/index.js:977:46)
    at process.processImmediate (node:internal/timers:483:21) {
  code: 'ERR_INVALID_ARG_TYPE'
}

Node.js v20.18.0