Testing an invalid argument w/ Chai in JavaScript

Preface: Last week I started tinkering with JavaScript, so this may or may not be a dumb question (though I'll wager the former).

I created this method to return the day of the week when getDayOfTheWeekFromDate is initialized with a Date(). I'm trying to create a test with chai to see what it does when it's fed a bogus argument (like a string).

Two questions:

1. Am I checking the type correctly?

2. How would I test a bogus argument for getDayOfTheWeekFromDate with chai?

   DateHelper.prototype.getDayOfTheWeekFromDate = function(inputDate) {
       if (inputDate instanceof Date) {
           inputDate = new Date();
           var dayOfTheWeek = inputDate.getDay();
   
           switch(dayOfTheWeek) {
               case 0: return "Sunday";
               case 1: return "Monday";
               case 2: return "Tuesday";
               case 3: return "Wednesday";
               case 4: return "Thursday";
               case 5: return "Friday";
               case 6: return "Saturday";
               default: return;
           }
       } else {
           // I'm shooting for doing nothing if the input isn't a Date()
           // How would I test this?
           return;
       }
   };
   
   

I've got a test class set up to test a Date helper class.

'use strict';
var chai = require('chai');
var expect = chai.expect;
var DateHelper = require('../date_helper');
chai.config.includeStack = true; // true enables stack trace

describe('DateHelper', function() {

    context('With a valid date value, getDayOfTheWeekFromDate', function() {
        it('should return Tuesday', function() {
            expect(subject.getDayOfTheWeekFromDate(new Date())).to.eq('Tuesday');
        });
    });
});

I tried this, but the console wigs out when I feed the initializer a string, doesn't know what undefined is. What should I be testing here?:

context('With a invalid date value, getDayOfTheWeekFromDate', function() {
    it('should return nothing', function() {
        expect(subject.getDayOfTheWeekFromDate('bogusArgument').to.be.undefined;
    });
});