Windows 8.1: "new AppCallbacks()" crashes when using advertising SDK

I upgraded to Unity 4.6, and now suddenly my app crashes here:

public App()
{
this.InitializeComponent();
appCallbacks = new AppCallbacks(false); <---- Crashes
appCallbacks.RenderingStarted += RemoveSplashScreen;
}

It first gets a first-chance exception with a TypeLoadException on AppCallbacks (“class not registered”), and then it gets a fatal WinRT exception and dies. What’s strange is that I only see this when my project has a reference to the 8.1 Advertising SDK. If I remove the reference, it works again. I tried creating a non-Unity 8.1 app and adding a reference to the ad SDK and it worked fine. Has anyone seen this problem before?

Here’s another thread I found on the same topic: http://forum.unity3d.com/threads/windows-8-1-system-typeloadexception-when-adding-sdks.283454/