NodeUnit - Testing an asynchronous function

I want to tests the results returned by an asynchronous function with nodeunit. I have the following test :

module.exports = {
  createFail: function(assert) {
    var user = new CRUDUser({ firstName: 'Node' });

    assert.equal(undefined, user.id);

    user.create(function(res) { // user.create() makes an async AJAX call and should call this function back
      assert.equal(false, res.success);
      assert.equal(undefined, user.id); // user not created cause 'lastName' can't be null

      console.log('#####');
      assert.done();
    });
  }
};

But when I run it I get the following error :

FAILURES: Undone tests (or their setups/teardowns): 
- createFail

To fix this, make sure all tests call test.done()

If I move the assert.done(); call outside the callback function, the test ends up before the AJAX call has been made.

I also tried to add assert.expect(3); at the very beginning of the test to make it "wait" until the callback function calls assert.done(); but I get the same error as above.

In all cases, the expected ##### isn't outputed to the console obviously. What am I doing wrong ??