Jest - TypeError: Cannot read property '[object Array]' of undefined

I've written a unit test in jest and am seeing this error when running it. I'm getting the proper data in the console. I've already reviewed these posts similar to mine, but still don't understand why it's happening:

• One
• Two
• Three

This is my test:

import filterAvailableSlots from './filterAvailableSlots';

/* UNIT TEST */

describe('filterAvailableSlots', () => {
  it('should return an array', async () => {
    const bookedSlots = { '10:30': true, '11:00': true };
    const allSlots = ['9:30', '10:00', '10:30', '11:00', '11:30', '12:00'];
    const availableSlots = filterAvailableSlots([allSlots, bookedSlots]);
    expect(Array.isArray(availableSlots)).toBe(true);
  });
});

This is my code:

/**
 * @param {Array} allSlots
 * @param {Object} bookedSlots
 * @return an array of available slots
 */

export default function filterAvailableSlots(allSlots, bookedSlots) {
  let availableSlots = [];
  availableSlots = allSlots.filter((item) => !bookedSlots[item]);
  return availableSlots;
}

Should be filtering an array of times, and removing any items that match a bookedSlots object key.

Image with more error detail:

doesn't like my reference to the object property

The correct return value, which I am seeing properly via console.log:

[ '9:30', '10:00', '11:30', '12:00' ] 

Must be a Javascript quirk I'm not understanding. Anyone have any idea why the test is failing even though I'm seeing the correct data in my console?