self.performSegueWithIdentifier not rendering?

I've got what I thought was a simple bit of code..

dispatch_async(dispatch_get_main_queue(), {
                        self.performSegueWithIdentifier("goToLogin", sender: nil)
                    })

This is called from a callback handler to a REST service. The identifier is correct, and a print() call in the viewDidLoad of my target view fires, so it seems the segue does in fact happen, but the view on-screen doesn't change.. I'm not sure what to make of this -- has anyone seen this before?