System.TypeLoadException Could not find Windows Runtime type 'Windows.Foundation' exception

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C#

Windows

XAML

In my Windows Phone 8.1 app I'm creating a CustomPage and then navigating to it, all in code behind. My CustomPage class is:

using Windows.UI.Xaml.Controls;

namespace TestApp {
	public class CustomPage : Page {
		public CustomPage() {
			this.BuildPage();
		}

		private void BuildPage() {
			var panel = new StackPanel();
			panel.Children.Add(new TextBlock { Text = "Hello World" });
			this.Content = panel;
		}
	}
}

And then in my MainPage.xaml.cs I'm doing this:

CustomPage myNewPage = new CustomPage();
Frame.Navigate(myNewPage.GetType());

However this throws a System.TypeLoadException exception - Could not find Windows Runtime type 'Windows.Foundation'
If I don't do a CustomPage class just a Page class, it works fine, but I need to create a CustomPage class, how could I solve this?