I'm stuck

var questions = [
['What color is the sky?', 'blue'],
['Whats the beginning of the Alphabet'], ['A'],
['When its hot people usually like to eat what?', 'ice cream']  
];

var count = 0;  // users input that are correct
var response; // the responses from user
var correct = []; //answers correctly 
var wrong =[];//answers that are wrong


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

for ( var i = 0; i < questions.length; i += 1) {
  response = prompt(questions[i][0]);
  answer = questions[i][1];
    if ( response.toLowerCase() === answer.toLowerCase() ){
      count += 1;
      correct.push(questions[i][0]);
     } else {
      wrong.push(questions[i][0]);
     }
}



after the second question being asked, whatever the responses is..it says 'caught TypeError: Cannot read property 'toLowerCase' of undefined'

what am i doing wrong?