downcast to AVAssetDownloadURLSession fails

Question

I'm creating an AVAssetDownloadURLSession

here's my code:

let config = URLSessionConfiguration.background(withIdentifier: "background")
let avSession = AVAssetDownloadURLSession(configuration: config, assetDownloadDelegate: self, delegateQueue: nil)

let any : Any = avSession
print("any as? AVAssetDownloadURLSession \(any as? AVAssetDownloadURLSession)") // always returns nil        

Now, if I print out avSession, it says it is an __NSURLBackgroundSession, not an AVAssetDownloadURLSession. If I assign avSession to another variable (in this case an Any), and then downcast it later, it fails.

This is a real problem because I do need to do exactly this. Why can't I downcast this without failing? If I inspect the memory address of avSession, and then inspect any in the debugger, they both have the same address. But it is most certainly (according to the debugger, and the runtime itself) not an AVAssetDownloadURLSession.

Thanks for any help with this.