Stub overriden toString() using Mockito / PowerMock

Question

I'm trying to stub a toString method of an abstract class that overrides it and make it final:

public abstract class ExampleClass
{
    private String someClassProperty;

    public final String toString()
    {
      return this.someClassProperty;
    }
}

The problem is that when I stub it as it's normally done with the following code:

ExampleClass mock = mock(ExampleClass.class);
when(mock.toString()).thenReturn("myString");

At runtime when Powermock is doing the stubbing on the mock it throws a NPE:

Method threw 'java.lang.NullPointerException' exception. Cannot evaluate ExampleClass$$EnhancerByMockitoWithCGLIB$$.toString()

I have put the ExampleClass in the @PrepareForTest annotation. So does anyone knows what actually can be wrong with stubbing final methods and if I make mistake somewhere. Thanks.