Quiz returning NaN values

var quiz = [
  ['How tall is David?', 6],
  ['What is the meaning of life?', 42],
  ['How many fingers are there on one human hand?', 5]

];
var correctAnswers  = 0;
var question;
var answer;
var response;
var html;
var correct = [];
var wrong = [];

function print(message) {
  document.write(message);
}

function buildList(arr) {
  var listHTML = '<ol>';
  for ( var i = 0; i < arr.length; i += 1 ) {
    listHTML += '<li>' + arr[i] + '</li>';
  }
    listHTML =+ '</ol>';
  return listHTML;
}

for ( var i = 0; i < quiz.length; i += 1) {
  question = quiz[i][0];
  answer = quiz[i][1];
  response = prompt(question);
  response = parseInt(response);
  if (response === answer) {
    correctAnswers += 1;
    correct.push(question);
  } else {
    wrong.push(question);
  }

}



html = "You got " + correctAnswers + " questions correct!";
html +=  '<h2>You got these questions correct:</h2>';
html += buildList(correct);
html += '<h2>You got these question incorrect:</h2>';
html += buildList(wrong);
print(html);