JavaScript error on quiz

[simpsonw]

If you go to the second quiz question ("Why do you need to reduce a research question to key concepts?") at:

http://ec2-52-3-120-211.compute-1.amazonaws.com/guide_on_the_side/tutorial/how-do-you-analyze-a-research-question

and choose an answer and click "Check Answer", a JavaScript error appears in the console:

Uncaught TypeError: Cannot read property 'replace' of undefined
    at Object.success (view_tutorial_only.js:216)
    at o (jquery.min.js:2)
    at Object.fireWith [as resolveWith] (jquery.min.js:2)
    at w (jquery.min.js:4)
    at XMLHttpRequest.d (jquery.min.js:4)