mockMvc POST method java.lang.AssertionError: Status Expected :200 Actual :400

I wrote a test using mockito. I have the following REST controller with this POST request

@RestController
@RequestMapping(FriendResource.FRIENDS)
public class FriendResource {

    public static final String FRIENDS = "/friends";

    private FriendService friendService;

    @Autowired
    public FriendResource(FriendService friendService) {
        this.friendService = friendService;
    }

    @PostMapping
    public FriendDto create(@Valid @RequestBody FriendDto friendDto) {
        return this.friendService.create(friendDto);
    }
}

Here the method of the service implementation for mapping and persistence

    @Override
    public FriendDto create(FriendDto friendDto) {
        Friend friend = this.friendPersistence.create(friendDto);
        return new FriendDto(friend);
    }

Here the test for this method:

@RunWith(MockitoJUnitRunner.class)
public class FriendResourceTest {

    private MockMvc mockMvc;

    @InjectMocks
    private FriendResource resource;

    @Mock
    private FriendService friendService;

    @Before
    public void setUp() {
        this.mockMvc = MockMvcBuilders
                .standaloneSetup(resource)
                .build();
    }

    @Test
    public void shouldReturnFriendDtoWhenRetrieveFriendDto() throws Exception {
        //given
        FriendDto validDto = new FriendDto("Tom", "666666666");
        when(friendService.create(validDto)).thenReturn(validDto);

        //when
        final ResultActions result = mockMvc.perform(
                post(FRIENDS)
                        .accept(MediaType.APPLICATION_JSON))
                .andDo(print());

        //then
        result.andExpect(status().isOk());
        result.andExpect(content().contentType(MediaType.APPLICATION_JSON));
        verify(friendService, times(1)).create(validDto);
    }

}

I tried this but it didn't work: verify(friendService, times(1)).create(refEq(validDto));

And the log:

MockHttpServletRequest:
      HTTP Method = POST
      Request URI = /friends
       Parameters = {}
          Headers = [Accept:"application/json"]
             Body = <no character encoding set>
    Session Attrs = {}

Handler:
             Type = com.user.rest.FriendResource
           Method = com.user.rest.FriendResource#create(FriendDto)

Async:
    Async started = false
     Async result = null

Resolved Exception:
             Type = org.springframework.http.converter.HttpMessageNotReadableException

ModelAndView:
        View name = null
             View = null
            Model = null

FlashMap:
       Attributes = null

MockHttpServletResponse:
           Status = 400
    Error message = null
          Headers = []
     Content type = null
             Body = 
    Forwarded URL = null
   Redirected URL = null
          Cookies = []

java.lang.AssertionError: Status 
Expected :200
Actual   :400
<Click to see difference>


    at org.springframework.test.util.AssertionErrors.fail(AssertionErrors.java:59)
    at org.springframework.test.util.AssertionErrors.assertEquals(AssertionErrors.java:122)
    at org.springframework.test.web.servlet.result.StatusResultMatchers.lambda$matcher$9(StatusResultMatchers.java:627)
    at org.springframework.test.web.servlet.MockMvc$1.andExpect(MockMvc.java:196)
    at com.user.business_services.FriendResourceTest.shouldReturnFriendDtoWhenRetrieveFriendDto(FriendResourceTest.java:117)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.mockito.internal.runners.DefaultInternalRunner$1$1.evaluate(DefaultInternalRunner.java:46)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
    ...

The post request returns an empty body and status 400. The other routs are passing the tests.

Thanks by the way.