Calling A method within an abstract class

I'm trying to fake an abstract class and call one its method and it does not work.

the abstract class has a method called : LoadObjectInSession(string key);

 

I tried the following and each and everytime it returns null ;

 

1)

var con = Isolate.Fake.AllInstances<BaseController>();

Isolate.WhenCalled(() => con.LoadObjectInSession("myKey")).WillReturn("aaa");

2)

MockObject<BaseController> mockInterface = MockManager.MockObject<BaseController>();

mockInterface.ExpectUnmockedCall("LoadObjectInSession");

Isolate.WhenCalled(() => mockInterface.Object.LoadObjectInSession("myKey")).WillReturn("aaa");

 

Please help me understand why everytime it's returning null ?

 

Thank you