Can't update question on click

//create Question
function Question(text, correct, mistake){
 this.text = text; 
 this.correct = correct;
 this.mistake = mistake;
}

//create QuestionSet
function QuestionSet(){
 this.questions = [];
 this.currentQuestion = 0;
 this.add = function(question){
  this.questions.push(question); 
 }
}

//add display functionality to questionSet
QuestionSet.prototype.displayQuestion = function(){
  var container = document.getElementById("question"),
      correctContainer = document.getElementById("choice0"),
      wrongContainer = document.getElementById("choice1");

  container.innerHTML = this.questions[this.currentQuestion].text;
  correctContainer.innerHTML = this.questions[this.currentQuestion].correct;
  wrongContainer.innerHTML = this.questions[this.currentQuestion].mistake;
}


//move to nextQuestion
QuestionSet.prototype.nextQuestion = function(){
  this.currentQuestion++;
  if(this.currentQuestion===this.questions.length){
   this.currentQuestion=0; 
  }
  this.displayQuestion();
}


//create questions
var q1 = new Question("What\'s my name?", "Jim", "Bob");
var q2 = new Question("What\'s my surname?", "Smith", "Johnson");
var q3 = new Question("What\'s my favourite snack?", "Chocolate", "Apples");

//create new QuestionSet
var set = new QuestionSet();

//add questions to QuestionSet
set.add(q1);
set.add(q2);
set.add(q3);

//display current Question
set.displayQuestion();



//update question on click
var button = document.getElementsByClassName('btn--default');
for(var i=0; i<button.length;i++){
 button[i].addEventListener('click',set.nextQuestion); 
}