iOS: @testable not seeing internal class in framework

This has been baffling me for days because I'm sure I've done it in the past. I have a framework (MyFramework) which contains a view controller:

import UIKit
public class PublicVC: UIViewController {
    /* In here we use InternalVC */
}

class InternalVC: UIViewController {}

And a MyApp target that uses this framework and refers to PublicVC.

In the test for the app, I have added the framework as a dependency so I can access the internal VC in the tests and checked that it's also being embedded just to be sure. Then done this:

@testable import MyFramework
@testable import MyApp

let public: PublicVC = // works
let internal: InternalVC = // Fails to compile !!! Unresolved Identifier

I'm sure I've access internal classes in frameworks in the past for testing, but for some reason this project won't let me see the class not matter what I try.

Anyone have any ideas what I've missed?