Ambiguous reference to member 'provider' (CXProvider) in Swift 3? (CallKit)

Question

I have compile error with this line of code:

self?.provider .reportOutgoingCall(with: callUUID, connectedAt: nil)

saying: Ambiguous reference to member 'provider'

Here is complete function:

        let callUUID = NSUUID (uuidString: callUUIDString!)
        let startCallAction = CXStartCallAction.init(call: callUUID as! UUID, handle: CallProvider.handleFromCall(_call: _call))
        startCallAction.isVideo = false

        callController .request(CXTransaction.init(action: startCallAction)) { [weak self] error in
            if error == nil {
                provider .reportOutgoingCall(with: callUUID, connectedAt: nil)
                startCallAction .fulfill()
            } else {
                startCallAction.fail()
            }
        }

In Objective-C, this works, but in swift 3 cannot build. Any answers? thx!