Content-Length Header is Incorrect

Not sure if this is an express issue or with http-proxy but I am modifying (extending) the response body and the resulting Content-Length header is not getting extended. When I try to extend it with proxyRes.headers this does not get passed on to the client and there is no res.headers method/hashmap.

const express = require('express');
var app = express();
var httpProxy = require('http-proxy');
var apiProxy = httpProxy.createProxyServer();

app.get('/', (req, res) => {
  apiProxy.web(req, res, {target: MYB});
});

apiProxy.on('proxyRes', (proxyRes, req, res) => {
  var _write = res.write;
  var body = '';
  proxyRes.on('data', function(data) {
    data = data.toString('utf-8');
    body += data;
  });
  res.write = function(data) {
    try {
      if (res.statusCode === 200 && proxyRes.headers['content-type'] === 'text/html') {
        // body.length is 411 bytes
        body = body.replace('</body>', '<hr>&copy; aaa 2022 Marc Cawood</body>');
        // body.length is now 425 bytes
        proxyRes.headers['content-length'] = Buffer.byteLength(body, 'utf-8');
        _write.call(res, body);
       // Browser still gets Content-Length: 411 :-(
      }
    } catch (e) {
      console.trace(e);
      console.error(e.message);
      throw e;
    }
  };
});