How to mock up the EntityManager using jmockit's Mockups API?

I have a problem with mocking up the EntityManager. Everything compiles, the test runs but the mocked method returns null.

When I set the breakpoint inside the mocked 'find' method the app is never suspended there. I managed to successfully mock different class with static methods this way - but with this one I have problems.

I use Jmockit 1.7 along with Java 1.8.0. The class I am trying to mock is: javax.persistence.EntityManager

If there are any more information needed - please ask. I would be very grateful for any help.

Here is my code:

@RunWith(JMockit.class)
public class ShapefileSerializerTest {

    @Mocked
    private EntityManager em;

    @BeforeClass
    public static void setUpBeforeClass() throws Exception {
        new MockDatabase();
    }

    @Test
    public void testPrepareShapefile() {
        String[][] data = new String[][] {{"1", "100"}, {"1", "101"}, {"1", "102"}, {"1", "103"}, {"2", "200"}, {"2", "201"}};

        List<Map<String, String>> featuresData = Stream.of(data).map(row -> {
            Map<String, String> map = new HashMap<>(2);
            map.put("layerId", row[0]);
            map.put("id", row[1]);
            return map;
        }).collect(Collectors.toList());

        ShapefileSerializer shapefileSerializer = new ShapefileSerializer("shapefiles");
        // if I do not set up the em here - then it will be null inside the tested class
        Deencapsulation.setField(shapefileSerializer, em);

        Response response = shapefileSerializer.prepareShapefile(featuresData);

        assertEquals(Status.OK.getStatusCode(), response.getStatus());
    } 

    public static final class MockDatabase extends MockUp<EntityManager> {
        @Mock
        @SuppressWarnings("unchecked")
        public <T> T find(Class<T> entityClass, Object primaryKey) {
            return (T) new ProjectLayer();
        }
    }
}