I need to insert a dash between two vowels if my string has two consecutive vowels, I have an error as it is returning null. 

```js
function insertDash(word) {
  const strWord = word.split('');
  for (let i = 0; i < strWord.length; i += 1) {
    if (strWord[i - 1].includes(/[aeiou]/gi) && strWord[1].includes(/[aeiou]/gi)) {
      strWord.push('-');
    }
  }
  return strWord.join('');
}

insertDash("Thiis is a teest");
```