freeCodeCamp: Caesars Cipher: My code pass all the tests but doesn't validate on FCC

Challenge Caesars Cipher has an issue. User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.106 Safari/537.36.

The code pass all the tests but doesn’t validate on FCC editor. Why that?

My code:

let engine = (v) => {

  let low = 'A'.charCodeAt();
  let high = 'Z'.charCodeAt();

  if (!!v.match(/[a-z]/i)) {
    if (v.charCodeAt() - 13 < low) {
      let code = (high - (low - (v.charCodeAt() - 13 + 1)));

      return String.fromCharCode(code);
    }
    else {
      return String.fromCharCode(v.charCodeAt() - 13);
    }
  }
  else {
    return v;
  }
};

function rot13(str) { // LBH QVQ VG!

  return '"' + str
    .split('')
    .map(engine)
    .join('') + '"';
}

// Change the inputs below to test
rot13("SERR PBQR PNZC");
rot13("SERR CVMMN!");
rot13("SERR YBIR?");
rot13("GUR DHVPX OEBJA QBT WHZCRQ BIRE GUR YNML SBK.");