WhenCalled applied in Session will Cause Exception

    public class DerivedController:System.Web.Mvc.Controller
    {
        
    }
    [TestFixture, ClearMocks]
    public class ControllerTest
    {
        [Test, Isolated]
        public void FirstController()
        {
            var controller = Isolate.Fake.Instance<DerivedController>();
            Isolate.WhenCalled(()=>controller.Session).ReturnRecursiveFake();
            Isolate.WhenCalled(() => controller.Session["a"])
                .WillReturn("a");

            Assert.IsNull(controller.Session["b"]);

        }
    }

failed: TypeMock.TypeMockException : 
*** Unexpected Call to System.Web.HttpSessionStateBase.get_Item()
   at b3.b(Object A_0, String A_1, String A_2, MethodBase A_3, Object[] A_4, Object A_5)
   at System.Web.HttpSessionStateBase.get_Item(String name)
   C:developmentEsteem7 .Net2ELockRecordWebSourceELockRecordWebELockRecordWebTestControllerControllerTest.cs(27,0): at ELockRecordWebTest.Controller.ControllerTest.FirstController()
   at TypeMock.MockManager.a(String A_0, String A_1, Object A_2, Object A_3, Boolean A_4, Object[] A_5)
   at TypeMock.InternalMockManager.getReturn(Object that, String typeName, String methodName, Object methodParameters, Boolean isInjected)

   Isolate.WhenCalled(() => controller.Session["a"])
                .WillReturn("a");