no response received when sending integer value as response to NodeJS API call

I am trying to send some integer value as response after making call to API Endpoint.
This is sample code to illustrate the problem:

app.post('/add5', (req, res) => {
  result = req.body.num + 5;
  res.send(result);
});

And this is the request payload:

{
  "num": 7
}

Expected output: 12

But following output logged in terminal and no(blank) response is received.

RangeError [ERR_HTTP_INVALID_STATUS_CODE]: Invalid status code: 12
    at new NodeError (node:internal/errors:371:5)
    at ServerResponse.writeHead (node:_http_server:272:11)
    at ServerResponse._implicitHeader (node:_http_server:263:8)
    at ServerResponse.end (node:_http_outgoing:871:10)
    at ServerResponse.send (/storage/emulated/0/codes/api/node_modules/express/lib/response.js:221:10)
    at /storage/emulated/0/codes/api/index.js:208:7
    at Layer.handle [as handle_request] (/storage/emulated/0/codes/api/node_modules/express/lib/router/layer.js:95:5)
    at next (/storage/emulated/0/codes/api/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/storage/emulated/0/codes/api/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/storage/emulated/0/codes/api/node_modules/express/lib/router/layer.js:95:5)

But, adding some string in the string works totally fine.

app.post('/add5', (req, res) => {
  result = req.body.num + 5;
  res.send("Result: " + result);
});

This returns expected output i.e. Result: 12

>Solution :

Use res.json(result) or res.send(JSON.stringify(result)) or res.send(200, result). Otherwise express assumes that you’re passing a http status code to send, as the error message signifies. Or upgrade to express 4, where this cannot happen any more.