JavaScript JavaScript Unit Testing Improving Our Tests Catching an Error

Yet another chai docs question (throwing an error)

So I thought this would be the right way to throw an error based on the video and previous examples. I really don't understand what I'm doing wrong here...

subtraction_spec.js
var expect = require('chai').expect

describe('subtraction', function () {
  var subtraction = require('./subtraction.js').subtraction;  
  it('only works with numbers', function () {
    // YOUR CODE HERE
    var handler = function () {subtraction("a", "b");};
    expect(handler).to.throw(Error);
    expect(handler).to.throw('subtraction only works with numbers!');
  });
});
subtraction.js
function subtraction (number1, number2) {
  if (typeof number1 !== 'number' || typeof number2 !== 'number') {
    throw Error('subtraction only works with numbers!')
  }
  return number1 - number2
}

module.exports.subtraction = subtraction;