Swift closure parameter with default value nil still returns some value when not passed by the caller?

Question

In this code:

@objc func postToken(completion: (()-> Void)? = nil) {
    someBlock {
        completion?()
    }
}

when added as a call using a selector, like so:

let tokenIndicatorGesture = UITapGestureRecognizer(target: self, action: #selector(postToken))

the closure is not nil and thus crashes at the completion?() line.

the value looks like:

completion  (())?   0x00000001049476f8 MyApp`partial apply forwarder for reabstraction thunk helper from @escaping @callee_unowned @convention(block) () -> () to @escaping @callee_guaranteed () -> () at <compiler-generated>

Searching for the error didn't return much so I am looking for some help figuring this out.