Keep Coding

I have a controller that returns a DeferredResult, it is a POST.
Controller:
#RequestMapping(value = "/{customerId}/cards", method = RequestMethod.POST)
#ResponseBody
public DeferredResult<CardSummaryQuery> createCard(
#PathVariable String customerId,
#RequestBody CustomerDto payload) {
DeferredResult<CardSummaryQuery> deferred = new DeferredResult<>();
Observable<CardSummaryQuery> observable =
cardService.registerCardObservable(customerId, payload);
observable.subscribe(deferred::setResult, deferred::setErrorResult);
return deferred;
}
I am trying to test it but receiving the following error: "java.lang.AssertionError: Content type not set"
MvcResult result = mockMvc
.perform(post("/customers/55e5d-4df4r-v4fgt/cards").content(payload)
.contentType(MediaType.APPLICATION_JSON))
.andExpect(status().isOk()).andExpect(request().asyncStarted())
.andExpect(request().asyncResult("Deferred result")).andReturn();
mockMvc.perform(asyncDispatch(result)).andExpect(jsonPath("$.success").value(true));
What can I do to solve?