Why is my custom Error object converted to a String by the Express router?

I am using custom Error objects in node.js (0.8.12) with express (2.5.8). When errors occur, I want to route them to my custom error handler, but my custom Error objects are converted to Strings, while native Error objects stay the same.

Here is a simple example:

var express = require('express');var APIError = function(type) {    this.type = type;}APIError.prototype = new Error();var app = express.createServer();app.use(app.router);app.use(apiErrorHandler);function log(source, err) {  console.log(    source,     typeof err,     err.constructor.name,     err instanceof APIError,     err instanceof Error  );}function apiErrorHandler(err, req, res, next) {  log("error handler:", err);}app.get('/', function(req, res, next) {  var err = new APIError("notAllowed");  log("router:", err);  next(err);});app.listen(80);

The console output from this example is the following:

router: object Error true trueerror handler: string String false false

If I replace new APIError("notAllowed") with new Error("notAllowed"), the object is conserved and a request produces this output:

router: object Error false trueerror handler: object Error false true

Why is my custom Error object converted, although it is an instance of Error?