测试learnyounode练习6时出现回调问题（使其模块化）

Question

在测试 learnyounode（使其模块化）的练习 6 时，我的代码通过了所有测试，但只有一个测试并显示 TypeError：回调不是函数。

  ACTUAL                                 EXPECTED                
────────────────────────────────────────────────────────────────────────────────

   "CHANGELOG.md"                      ==    "CHANGELOG.md"                     
   "LICENCE.md"                        ==    "LICENCE.md"                       
   "README.md"                         ==    "README.md"                        

────────────────────────────────────────────────────────────────────────────────

 ✓ 

 Submission results match expected

Error
 ✓ 

 Additional module file exports a single function

 ✓ 

 Additional module file exports a function that takes 3 arguments

 ✓ 

 Additional module file handles errors properly

 ✓ 

 Additional module file handles callback argument

 ✓ 

 Additional module file returned two arguments on the callback function

 ✓ 

 Additional module file returned an Array as the second argument of the
 callback

data.dat
learnyounode.dat
w00t.dat
words.dat
 ✓ 

 Additional module file correctly handles '.'-prefixed extension

 ✗ 

 Your additional module file [mymodule.js] did not return an Array with the
 correct number of elements as the second argument of the callback

 # FAIL Your solution to MAKE IT MODULAR didn't pass. Try again!

─────────────────────────────────────────────────────────────────────────────

mymodule.js：

const fs = require('fs')
const path = require('path');

function extFunc(directory, ext, callbackFunc) {
    fs.readdir(directory, function(err, files) {
        if (err) {
            console.log("Error")
            return callbackFunc(err);
        }
        
        files.forEach(function(file) {
            if (path.extname(file) === ext) {
                console.log(file);
            };
        });
        callbackFunc(err, files);
    })
};

module.exports = extFunc;

make-it-modular.js

const extFunc = require('./mymodule');

const directory = process.argv[2];
const ext = '.' + process.argv[3];

extFunc(directory, ext);

如果我能得到一些关于为什么 mymodule.js 中的回调函数抛出此错误并导致我最终无法通过 learnyounode 测试套件的反馈，我将不胜感激。