Mocha async test handle errors

I'm trying to create a test case with Mocha but my code is asynchronous.

That's fine, I can add a "done" callback function to "it" and that will work perfectly fine for positive cases. But when trying to test negative cases, it will just make the test fail.

I would like to make something like this but asynchronous:

someObject.someMethod(null).should.equal(false)

Instead I can only test for a callback to return, instead of testing what really happend (null is not valid):

it('this should return false or an error', function(done) {
    someObject.someMethod(null, '', done);
});

I would like to write something like this:

it('this should return false or an error', function(done) {
    someObject.someMethod(null, '', done).should.throw();
});

but that would lead to this error:

"TypeError: Cannot read property 'should' of undefined"

I also tried using expect and assert, but the same rules apply.

Any clues? Thanks