UUID value is serialized into different form in Spring test method

I am writing tests for my REST API controller and I need to check UUID value from returned JSON object, please see this test method:

@Test
public void findById() throws Exception {

    final String uuidString = "6c2b1c8a-3c29-4160-98b0-b8eaea7ea4d1";
    final UUID id = UUID.fromString(uuidString);
    final Envelope envelope = createEnvelope(id);

    when(envelopeService.findOne(id, currentUser)).thenReturn(Optional.of(envelope));
    when(utilService.getLoggedInUser()).thenReturn(currentUser);

    mockMvc.perform(get("/api/envelopes/{id}", uuidString)).
            andExpect(status().isOk()).andExpect(content().contentType(Util.APPLICATION_JSON_UTF8))
            .andExpect(jsonPath("$..id", is(uuidString)));

    verify(envelopeService, times(1)).findOne(id, currentUser);
    //verifyNoMoreInteractions(envelopeService);

}

but the test produces this error:

 Expected: is "6c2b1c8a-3c29-4160-98b0-b8eaea7ea4d1"
         but: was <["6c2b1c8a-3c29-4160-98b0-b8eaea7ea4d1"]>
        at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
        at org.springframework.test.util.JsonPathExpectationsHelper.assertValue(JsonPathExpectationsHelper.java:74)
        at org.springframework.test.web.servlet.result.JsonPathResultMatchers$1.match(JsonPathResultMatchers.java:86)
        at org.springframework.test.web.servlet.MockMvc$1.andExpect(MockMvc.java:171)

It seems that ID 6c2b1c8a-3c29-4160-98b0-b8eaea7ea4d1 is returned correctly but is in serialized into different structure.

What is wrong with my code?