MAKE IT MODULAR fails due to "wrong" order of function parameters

[pnahratow]

• node version: v6.5.0
• npm version: 3.10.7
• learnyounode version: 3.5.5

Error output

  ✗  Your additional module file [readdir.js] does not appear to pass back  
  an error received from fs.readdir(). Use the following idiomatic Node.js  
  pattern inside your callback to fs.readdir(): if (err) return  
  callback(err) 

My Code

 //// file: program.js
 const readdir = require("./readdir")
 const args = [...process.argv].slice(2);

 readdir(args[0], (err, data) => {
     if (err) {
         return console.log(err);
     }
     data.forEach((val) => console.log(val));
 }, args[1]);

 //// file: readdir.js
 module.exports = (dirname, callback, ext) => {
    const fs = require("fs");
    const path = require("path");

    fs.readdir(dirname, (err, data) => {
        if (err) {
            return callback(err);
        }

        const filteredDir = data.filter((val) => {
            if (ext) {
                return path.extname(val) === "." + ext;
            } else {
                return true;
            }
        })

        callback(null, filteredDir);
    })
}

Notes

Producing an error manually works as intended

philipp@debian-acer5750g ~/devel/js/fcc/lynode % node program.js madeupdirectory
{ Error: ENOENT: no such file or directory, scandir 'madeupdirectory'
    at Error (native)
  errno: -2,
  code: 'ENOENT',
  syscall: 'scandir',
  path: 'madeupdirectory' }

Problem

The check seems to fail due to the unexpected order of function parameters (dirname, callback, ext).
I wrote it this way because I thought it might be useful if I can just omit the ext parameter.
When changing the order of parameters to (dirname, ext, callback) the test passes.
I think the code I provided should pass too