Mockito mocking abstract method in parent class calls real method

I want to test a class where I want to mock one of its method calls, getMessage(). This is a protected method in its parent abstract class. The problem I am facing is that on the line I declare the mock (Mokito.when()), it actually goes ahead and calls the real method.

ClassUnderTest partialMockClassUnderTest = Mockito
        .spy(new ClassUnderTest());             

                Mockito.when(
        partialMockClassUnderTest
                .getMessage())
                .thenReturn("Mock Message");