Cannot set inner.HTML of null Driving me nuts!

var questions = [
    ["what year was the latest huge housing market crash?", 2008],
    ["How many hours are in a day?", 24],
    ["How many days are in a week?", 7]
];

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

function print(message) {
  var outputDiv = document.getElementById('output');
  outputDiv.innerHTML = 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 < questions.length; i += 1) {
    question = questions[i][0];
    answer = questions[i][1];
    response = parseInt(prompt(question));

   if (response === answer) {
        correctAnswers += 1;
         correct.push(question);

    } else {
        wrong.push(question);

    }


}

html = "You got " + correctAnswers + " question(s) right.";
html += '<h2>You got these questions correct:</H2>';
html += buildList(correct);
html += '<h2>You got these questions wrong:</H2>';
html += buildList(wrong);
print(html);