FakeItEasy: MissingMethodException - Cannot create an instance of an interface when calling A.Fake when debugging

when I debug MSUnit test code I get

System.MissingMethodException occurred
  Message=Cannot create an instance of an interface.
  Source=mscorlib
  StackTrace:
       at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck)
       at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache)
  InnerException: 

on the first A.Fake<ITest>() call

ITest tt = A.Fake<ITest>();

It runs normally when I just run tests. so it happens only under visual studio debugger. what is is missing?

windows7/VS2010 ultimate/FakeItEasy 1.7.4626.65 Started recently… not sure what changed.