Mocking a Protocol with a Generic Func

So I have the following Swift protocols...

protocol MyMessage {
    func encodeMessage() -> [String:AnyObject]
}

protocol Sender {
    func send<T:MyMessage>(id:Int,event:T)
}

And I am using them just fine in some code I wrote. Now I want to write some unit tests and mock the Sender protocol. Basically I want to capture the id and the event so I can check them in a unit test in which I pass a Sender into the class under test. So I started writing the mock and I got this far before I realized it wasn't going to work.

class MockSender<U : MyMessage> : Sender {
    var sentEvent : U?
    var sentId : Int?

    func send<T:U>(id:Int,event:T) {
        sendId = id
        // save message so I can verify it
        sentEvent = event
    }
}

The problem is that I can't figure out how to tie the type of U to the type of T. The compiler says U is not a protocol, which makes sense. If I replace U with T in the MockSender signature the sendEvent = event line gets flagged with the error that I can't assign something of type T to something of type _?. From what I know about Swift generics I understand why I am getting these errors. What I would like to know is if there is someway I can do what I am trying to do, basically mock the protocol with the generic function and capture the value so I can verify it.