Application won't start in release mode :"Can't find custom attr constructor image: System.Core.dll"

I'm attempting to create a release version of an app built on mono-android-4.10.01073. When I build and deploy in release mode I get the following error, and the app dies :

Loaded assembly: AWOL.dll
Loaded assembly: GoogleAdMob.dll
Loaded assembly: Xamarin.Mobile.dll
Loaded assembly: Mono.Android.dll [External]
Loaded assembly: System.Core.dll [External]
In mgmain JNI_OnLoad
11-20 23:30:48.284 W/        ( 6423): Can't find custom attr constructor image: System.Core.dll mtoken: 0x0a000fb6
11-20 23:30:48.284 F/        ( 6423): * Assertion at /Users/builder/data/lanes/monodroid-mlion-monodroid-4.10.1-branch/d23a19bf/source/mono/mono/metadata/class.c:5594, condition `!mono_loader_get_last_error ()' not met
11-20 23:30:48.284 E/mono-rt ( 6423): Stacktrace:
11-20 23:30:48.284 E/mono-rt ( 6423): 
11-20 23:30:48.284 E/mono-rt ( 6423):   at <unknown> <0xffffffff>
11-20 23:30:48.284 E/mono-rt ( 6423):   at MonoDroid.Utils.JniType.ToJniName (System.Type,Java.Interop.ExportParameterKind) <IL 0x00067, 0x0029b>
11-20 23:30:48.284 E/mono-rt ( 6423):   at MonoDroid.Utils.JniType.ToJniName (System.Type) <IL 0x00002, 0x00047>
11-20 23:30:48.284 E/mono-rt ( 6423):   at Java.Interop.TypeManager.RegisterType (string,System.Type) <IL 0x00001, 0x0006b>
11-20 23:30:48.284 E/mono-rt ( 6423):   at Android.Runtime.TypeManager.RegisterType (string,System.Type) <IL 0x00002, 0x0004b>

Everything works fine in debug mode, and the app will start if I set Linking to 'None' and Skip linking assemblies to 'mscorelib;System'

This is a bit of a bugger.

Any ideas anybody?