Stuck on Test-Driven Development with JavaScript

I am completely stuck in Learn TDD With Mocha, 4/1.

The error I’m getting is Did you set three const variables and describe the intended behavior of them using assert.equal?, but here is my code:

const assert = require('assert');
const Calculate =  require('../index.js')

describe('Calculate', () => {
  describe('.sum',() => {
    it('returns the sum of an array of numbers', () => {
      const inputArray = [1, 2, 3]
      const expectedResult = 6
      const result = Calculate.sum(inputArray)
      assert.equal(result, expectedResult)

I believe it is right, the code seems pretty valid and the test passes, but it is not being accepted. What is wrong with it?