Uncaught TypeError: Cannot set property 'innerHTML' of null

I've been trying to recreate this challenge from scratch. So far, so good, however im getting a console error. It claims that the error is starting somewhere at the bottom of the code (where the html is being written out).

I would post this in a nice screenshot but every time I add a comment (like this) asking how one does 'post a screenshot of the code' it gets overlooked.

anyway, blob follows.

function print(message) { var outputDiv = document.getElementById(output); outputDiv.innerHTML = message; }

var question; var answer; var userResponse; var userAnswer; var correctAnswer = 0; var wrongAnswer = 0; var correct = []; var incorrect = []; var html;

var quiz = [ ['How many states are in the United States?', 50], ['How many legs does an insect have?', 6], ['How many continents are there?', 7] ];

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]; userResponse = prompt( question ); userAnswer = parseInt( userResponse ); if ( userAnswer === answer ) { correctAnswer += 1; correct.push( question ); } else { wrongAnswer += 1; incorrect.push( question ); }
}

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