The script continues past 10 despite the while loop having guessCount < 10. Help?

var randomNumber = getRandomNumber(10);
var guess;
var guessCount = 0;
var correctGuess = false;

function getRandomNumber( upper ) {
  var num = Math.floor(Math.random() * upper) + 1; 
  return num;
}

while ( guessCount < 10 ) {
  guess = prompt('I am thinking of a number between 1 and 10. What is it?');
  guessCount += 1;
  if (parseInt(guess) === randomNumber) {
    correctGuess = true;
    break;
  }
}  

if ( correctGuess ) {
  document.write('You guessed the number!');
  document.write('It took you ' + guessCount + ' tries to guess the number ' + randomNumber);

} else {
    document.write('<h1>Sorry.  You did not guess the number.</h1>');
}