Can't pass the tests, that didn't understand (Dates)

Got simple task: name the season of the year according to the input date. Wrote simple code:

function getSeason(date) {
  const isValidDate = (d) => {
    return d instanceof Date && !isNaN(d);
  };
  let season = "";
  if (date === undefined) return "Unable to determine the time of year!";
  if (!isValidDate(date)) throw new Error("Invalid date!");

  const springStart = new Date(date.getFullYear(), 2, 1);
  const summerStart = new Date(date.getFullYear(), 5, 1);
  const autumnStart = new Date(date.getFullYear(), 8, 1);
  const winterStart = new Date(date.getFullYear(), 11, 1);

  if (date >= springStart && date < summerStart) {
    season = "spring";
  } else if (date >= summerStart && date < autumnStart) {
    season = "summer";
  } else if (date >= autumnStart && date < winterStart) {
    season = "autumn";
  } else if (date >= winterStart && date < springStart) {
    season = "winter";
  } else {
    return "winter";
  }
  return season;
}

Passes all of test, except the last two:

     it.optional('throws an error with message "Invalid date!" on tricky moment', function () {
                const fakeDate = {
                    toString() {
                        return Date.prototype.toString.call(new Date());
                    },
                    [Symbol.toStringTag]: 'Date'
                };
    
                Object.setPrototypeOf(fakeDate, Object.getPrototypeOf(new Date()));
    
                const res = checkForThrowingErrors.call(this, [
                    () => getSeason(fakeDate)
                ], 'Invalid date!');
    
                assert.strictEqual(res.every($ => $ === CORRECT_RESULT_MSG), true);
            });

Tryin to check prototypes, getPrototype and everything else. What the hell it wants from me? Got actual: false; expected: true on both. The getSeason() func sould return string (it's ok)