Unable to Isolate.Invoke an event using a null argument

[TestClass, Isolated]
public class Tests
{
   public class A
   {
      public event Action<string> StringEvent;
      public void CallStringEventWithNull()
      {
         StringEvent(null);
      }
   }

   [TestMethod]
   public void Passes()
   {
      A a = new A();
      a.StringEvent += (string str) => { };
      a.CallStringEventWithNull();
   }

   [TestMethod]
   public void Fails()
   {
      A a = new A();
      a.StringEvent += (string str) => { };
      Isolate.Invoke.Event(() => a.StringEvent += null, null);
   }

   [TestMethod]
   public void InterestinglyPasses()
   {
      A a = new A();
      a.StringEvent += (string str) => { };
      Isolate.Invoke.Event(() => a.StringEvent += null, (string)null);
   }
}