Please why i am getting this error from the console. "Uncaught TypeError: Cannot read property 'toUpperCase' of null"

// two dimensional array
var setQuestionsAndAnswers = [["What year was 9/11?", "2001"],["What year was Sadam Hussein Killed?", "2006"], ["Who is the 44th President of USA?", "BARACK OBAMA"],["Which year was slavery abolished in USA", "1865"], ["Who invented electricity?","THOMAS  EDISON"], ["What is the tallest mountain in the world?", "EVEREST"]]

// Declared 2 arrays to keep track of questions answered correctly and wrongly
var correctlyAnswered = [];
var wronglyAnswered = [];
var answer = "";


// iterating through the array
  for ( var i = 0; i < setQuestionsAndAnswers.length; i++) {
    answer = prompt(setQuestionsAndAnswers[i][0]);
    answer = answer.toUpperCase(); // im trying to change the "answer" to upper case but it won't let me.

    /** checking if the "answer" is equal to the second index 
    in each of the nested arrays  or not then i add that questions
    to the arrays i declared early respectively" **/
    if ( answer === setQuestionsAndAnswers[i][1]) {
      correctlyAnswered.push(setQuestionsAndAnswers[i][0]);
    }
    else if (answer !== setQuestionsAndAnswers[i][1]) {
      wronglyAnswered.push(setQuestionsAndAnswers[i][0]);
    }
  }