Testing onClick event on react component using Jasmine

I have a React component that has buttons for which the onClick events are bound to functions passed as properties from the parent component, e.g. <Counter counter=0 incrementCounter={incrementFunction} decrementCounter={decrementFunction} />.

I'm new to unit testing React components so I'm trying to check that the function gets called when clicking on the increment/decrement buttons. For this I'm using Jasmine's spyOn method, but it never seems to catch the function being called.

If I output a console.log in the function I'm using (e.g. let incrementCounter = () => { console.log("increment!"); };) then I can tell the function is being called when I do TestUtils.Simulate.click(incrementButton);, however the test still won't pass. What am I missing?

Counter.js

import React, { Component, PropTypes } from "react";

class Counter extends Component {

  constructor(props) {
    super(props);
  }

  render() {
    const { incrementCounter, decrementCounter, counter } = this.props;
    return (

        <div>
          <h1>Counter</h1>
          <p>
            <b>Counter: {counter} times</b>
            {" "}
            <button onClick={incrementCounter}>+</button>
            {" "}
            <button onClick={decrementCounter}>-</button>
          </p>
        </div>

    );
  }
}

Counter.propTypes = {
  incrementCounter: PropTypes.func.isRequired,
  decrementCounter: PropTypes.func.isRequired,
  counter: PropTypes.number.isRequired
};

export default Counter;

Counter.test.js

import React from "react";
import TestUtils from "react/lib/ReactTestUtils";
import Counter from "./Counter"

describe("Counter", function () {

  let renderedComponent = {};
  let heading = {};
  let decrementButton = {};
  let incrementButton = {};
  let incrementCounter = () => {};
  let decrementCounter = () => {};
  let counter = 0;

  beforeEach(function(){

    renderedComponent = TestUtils.renderIntoDocument(
        <Counter incrementCounter={incrementCounter} decrementCounter={decrementCounter} counter={counter} />
    );

    heading = TestUtils.findRenderedDOMComponentWithTag(renderedComponent, "h1");

    let buttons = TestUtils.scryRenderedDOMComponentsWithTag(renderedComponent, "button");

    decrementButton = buttons[1];
    incrementButton = buttons[0];

    this.incrementCounter = incrementCounter;

  });

  it("renders without problems", function () {

    expect(TestUtils.isDOMComponent(heading)).toBe(true);
    expect(heading.innerText).toMatch(/Counter/g);

    expect(TestUtils.isDOMComponent(decrementButton)).toBe(true);
    expect(decrementButton.innerText).toMatch(/-/g);

    expect(TestUtils.isDOMComponent(incrementButton)).toBe(true);
    expect(incrementButton.innerText).toMatch(/\+/g);

  });

  it("fires the increment function", function() {

    spyOn(this, "incrementCounter");
    TestUtils.Simulate.click(incrementButton);
    expect(this.incrementCounter).toHaveBeenCalled(); // Error: fuction doesn't get called

  });

});