iPad PopToRootAsync() results in System.NullReferenceException

Forms : 1.4.0.6341

I'm working on an app in which the user navigates through a number of ContentPages before reaching a MasterDetailPage. In order to navigate back to the start I'm invoking PopToRootAsync() from the MasterDetailPage. The application works fine when deployed to an iPhone but crashes on iPad with a System.NullReferenceException - stack trace as follows:

at Xamarin.Forms.Platform.iOS.TabletMasterDetailRenderer.ViewDidDisappear (Boolean animated) [0x00000] in <filename unknown>:0 
  at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, IntPtr principal, IntPtr delegate) [0x00005] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:62 
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00038] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:46 
  at com.pocketobjects.SmartPractise.iOS.Application.Main (System.String[] args) [0x0000c] in /Users/simon/Desktop/PocketObjects/Xamarin/CWD/iOS/Main.cs:20

Any ideas?