Interface mocking works improperly

I'm writing a test and trying to mock an interface IMyResponse. But it returns int after mocking. Here is what I do:

public interface IMyResponse extends Serializable {
    // ...
    Object getResponseObject();
}

private IMyResponse myResponse;
@Before
    public void setUp(){
    myResponse = mock(IMyResponse.class);
}
@Test
public void mockTest(){
    // myResponse.getResponseObject() is 0
    when(myResponse.getResponseObject()).thenReturn("value");
    // myResponse.getResponseObject() is 46
}

What am I doing wrong?