_iteratorError undefined is not a function

The following code takes the content of a file, replace some characters, and outputs the result:

test.txt:

# Title

## Title 2

Paragraph

index.js:

#!/usr/bin/env node

'use strict'

var fs = require('fs')
  , filename = process.argv[2]

if (process.argv.length < 3) {
  console.log('Usage: node ' + process.argv[1] + ' FILENAME')
  process.exit(1)
}

function massReplace(text, replacementArray) {
  let results = text
  for (let [regex, replacement] of replacementArray) {
    results = results.replace(regex, replacement)
  }
  return results
}

function transformHeadings(text, orig) {
  return massReplace(text,
     [/^## (.*)/gm,     '<h2>$1</h2>'],
     [/^# (.*)/gm,      '<h1>$1</h1>'] ]
  )
}

fs.readFile(filename, 'utf8', function(err, data) {
  if (err) throw err

  data = data.split(/\n\n/gm)
  var tree = data.slice()

  console.log(transformHeadings(data, tree))
})

I get this error:

alex@alex-K43U:~/node/m2n$ babel-node index4.js test.txt
/home/alex/node/m2n/index4.js:41
        throw _iteratorError;
              ^
TypeError: undefined is not a function
    at massReplace (/home/alex/node/m2n/index4.js:17:4)
    at transformHeadings (/home/alex/node/m2n/index4.js:30:2)
    at /home/alex/node/m2n/index4.js:39:3
    at fs.js:336:14
    at FSReqWrap.oncomplete (fs.js:99:15)

No idea what the problem is, nor what throw _iteratorError means.

I'm using Babel to parse the ES6 code.

What could be the problem?