JenkinsPipelineUnit - unable to mock getId() called within script block

@RunWith(MockitoJunitRunner.class)
class FooTest {
    @Mock // Same as "someThing = Mockito.mock(SomeThing.class)"
    private SomeThing someThing,

    private final Foo foo;

    @Before
    public void setup() throws Exception {
        foo = new Foo(); // our instance of Foo we will be testing
        foo.setSomeThing(someThing); // we "inject" our mocked SomeThing
    } 

    @Test
    public void testFoo() throws Exception {
        when(someThing.compute(anyInt()).thenReturn(2); // we define some behavior
        assertEquals(2, foo.bar(5)); // test assertion
        verify(someThing).compute(7); // verify behavior.
    } 
}