Segue build with errors (Xamarin Tutorial)

I'm a newb at Xamarin and app development. I'm trying to work through Xamarin's Hello, iOS Multiscreen tutorial. It says to add this to my code:

var callHistoryController = segue.DestinationViewController as CallHistoryController;

When I run build, the first line gets the error "Cannot convert type 'UIKit.UIViewController' to 'TestApp.CallHistoryController' via a built-in conversion"

My .cs file is labeled CallHistoryController as it should be.

Any help would be great.

Thanks.