How can I make a promise reject for testing?

I have a db.js set up to do all my database calls. This is an example of one of the functions that query the database.

db.getUserCount = function () {
return new Promise (function (resolve, reject) {
    db.users.count().then(function (result) {
        resolve (result);
    }, function(e) {
        reject (e);
    });
});

};

I am pretty new to JavaScript and testing. I have used mocha and chai to test that it resolves like this:

describe('getUserCount', function() {
    it('should be fulfilled when called', function() {
        return db.getUserCount().should.be.fulfilled; 
    });
});

How can I test the reject part of the promises. Do I have to use something like sinon or is there some simple way to make the promise fail?