Error While Trying to Emit an Error

The fn variable can be undefined so the function was wasn't emitting err as intended but instead was failing with `TypeError: Cannot read property 'length' of undefined`.

*Note*: that this change applied to the npm package 0.18.2 (cdbf2a5bb2a32e85) works, but I am having other troubles with master as it is. Perhaps this scenario is no longer possible. I just can't get to that point yet to test it.