Strange problem - can't work it out. Help!

Hi everyone. My code looks like this at the moment:

function randomGenerator(number1, number2) {
  if (number1 < number2) {
    var low = number1;
    var high = number2;
  } else {
    var low = number2;
    var high = number1;
  }
  var randomNumber = Math.floor(Math.random ()* (high - low) + 1 + low);
  return randomNumber;
}
alert ('Your random number is ' + randomGenerator(parseInt(prompt('Choose your first number.')), parseInt(prompt('Choose your second number.')) + '.'));

The last line is a bit of a mess, but I just wanted to see if I could use prompts to pass arguments to the randomGenerator function.

It all seemed to work, but I've across a problem...

When using prompts to pass arguments, and the number2 parameter is lower than number1 (ie. number1 = 10, number2 = 1), it says that the returned value is not a number when the function is called.

It's fine the other way round, and when you pass arguments to the function as numbers, e.g., alert(randomGenerator(10, 1)); it also works fine.

What's going on? I can't work it out and it's really annoying me.

Thanks in advance!