How to cast nil to IOKit.IOHIDValueCallback in swift

I write a small utility in swift which registers a callback using IOHIDManagerRegisterInputValueCallback. I am trying to be a good citizen and cleanup after myself.

The [documentation] (https://developer.apple.com/library/content/documentation/DeviceDrivers/Conceptual/HID/new_api_10_5/tn2187.html#//apple_ref/doc/uid/TP40000970-CH214-SW61) says that to unregister one should call the registration function with Null

Note: To unregister pass NULL for the callback.

unfortunately

IOHIDManagerRegisterInputValueCallback( hidManager, nil , nil);

doesn't compile saying :

Nil is not compatible with expected argument type 'IOHIDValueCallback' (aka '@convention(c) (Optional, Int32, Optional, IOHIDValue) -> ()')

How can I cast nil to the correct type ?

Here is a sample code which should compile :

import Foundation
import Carbon
import IOKit
import IOKit.usb
import IOKit.hid
class IOEventManager{
  func start()->Void{
    let hidManager = IOHIDManagerCreate( kCFAllocatorDefault, IOOptionBits(kIOHIDOptionsTypeNone) );
    let context = UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque());
    IOHIDManagerRegisterInputValueCallback( hidManager, nil, context);
  }
}

if you want a full project, checkout https://github.com/jeantil/autokbisw and look at IOKeyEventMonitor#deinit