What is the main cause of the Swift "Unable to trigger a closure" error?

Question

The following code (shot for brevity):

private lazy var googleViewController = { return GoogleSignInViewController() }() private lazy var timerViewController = { return TimerViewController() }() 

causes this error:

MainViewController.swift: 26: 6: Unable to cause closure of type '() → UIViewController' using argument list of type '()'

Explicitly pasting one of my view controllers fixes the problem:

 private lazy var timerViewController: TimerViewController = { return TimerViewController() }() 

However, I still do not understand the cause of the error and why this corrects the situation. I suppose this is somehow related to the type of output, but not sure about the details. What happens to my code?

I understand that I do not need to use closures in the code above, this means illustrative code