Ошибка при тестировании Filter

когда тестирую CharsetFilter вылетает ошибка

java.lang.NullPointerException: Cannot invoke "javax.servlet.http.HttpServletRequest.getCharacterEncoding()" because "this.request" is null
 

@RunWith(MockitoJUnitRunner.class)
class CharsetFilterTest {

    CharsetFilter filter;

    @Mock
    FilterChain chain;

    @Mock
    HttpServletRequest request;

    @Mock
    HttpServletResponse response;

    @Before
    public void setUp() throws Exception {
        filter = new CharsetFilter();
    }
    @Test
    public void doFilterSetEncodingToRequest() throws ServletException, IOException {

        when(request.getCharacterEncoding()).thenReturn("UTF-8");

        filter.doFilter(request, response, chain);
        assertEquals("UTF-8", request.getCharacterEncoding());
    }
}

что можете посоветовать?