UNetWeaver problem on upgrade from 5.1 to 5.2

Unity Engine
,
1

Hi, currently trying to upgrade from 5.1.2f1 to 5.2.1p2
I get the following error and cannot run the game. Anyone seen this before ?

UNetWeaver error: Exception :System.NullReferenceException: Object reference not set to an instance of an object
  at Unity.UNetWeaver.Weaver.IsDerivedFrom (Mono.Cecil.TypeDefinition td, Mono.Cecil.TypeReference baseClass) [0x00000] in C:\buildslave\unity\build\Extensions\Networking\Weaver\UNetWeaver.cs:1365
  at Unity.UNetWeaver.NetworkBehaviourProcessor.ProcessSyncVars () [0x00236] in C:\buildslave\unity\build\Extensions\Networking\Weaver\UNetBehaviourProcessor.cs:1811
  at Unity.UNetWeaver.NetworkBehaviourProcessor.Process () [0x0001c] in C:\buildslave\unity\build\Extensions\Networking\Weaver\UNetBehaviourProcessor.cs:47
  at Unity.UNetWeaver.Weaver.ProcessNetworkBehaviourType (Mono.Cecil.TypeDefinition td) [0x0008b] in C:\buildslave\unity\build\Extensions\Networking\Weaver\UNetWeaver.cs:966
  at Unity.UNetWeaver.Weaver.CheckNetworkBehaviour (Mono.Cecil.TypeDefinition td) [0x0008d] in C:\buildslave\unity\build\Extensions\Networking\Weaver\UNetWeaver.cs:1444
  at Unity.UNetWeaver.Weaver.Weave (System.String assName, IEnumerable`1 dependencies, IAssemblyResolver assemblyResolver, System.String unityEngineDLLPath, System.String unityUNetDLLPath, System.String outputDir) [0x00098] in C:\buildslave\unity\build\Extensions\Networking\Weaver\UNetWeaver.cs:1553
UnityEngine.Debug:LogError(Object)
Unity.UNetWeaver.Log:Error(String) (at C:/buildslave/unity/build/Extensions/Networking/Weaver/Program.cs:20)
Unity.UNetWeaver.Weaver:WeaveAssemblies(IEnumerable`1, IEnumerable`1, IAssemblyResolver, String, String, String) (at C:/buildslave/unity/build/Extensions/Networking/Weaver/UNetWeaver.cs:1644)
Unity.UNetWeaver.Program:Process(String, String, String, String[], String[], IAssemblyResolver, Action`1, Action`1) (at C:/buildslave/unity/build/Extensions/Networking/Weaver/Program.cs:34)
UnityEditor.Scripting.Serialization.Weaver:WeaveUnetFromEditor(String, String, String, String, Boolean)
2

Actually was the same problem as this

A better error reporting would be nice.