Spring Cloud Contract generated tests fails on empty responses from producer

Question

I am trying to implement Spring Cloud Contract to my project. I am following instructions from this baeldung article: https://www.baeldung.com/spring-cloud-contract

• Dependencies are added
• Plugin is configured
• Producer contract is defined
• BaseTest is defined

Unfortunately my generated tests fails because the response (jsonBody) is "empty"

Here's a few pieces of the setup:

BaseContractTest =>

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.MOCK)
@AutoConfigureMessageVerifier
@DirtiesContext
@AutoConfigureStubRunner(ids = "com.example:producer-service:+:stubs:8080",
    stubsMode = StubRunnerProperties.StubsMode.LOCAL)
public class BaseContractTest
{
  @Autowired
  private WebApplicationContext webApplicationContext;
  
  @BeforeEach
  void setUp()
  {
    final DefaultMockMvcBuilder defaultMockMvcBuilder =
        MockMvcBuilders.webAppContextSetup(webApplicationContext);

    defaultMockMvcBuilder.apply(
        springSecurity((request, response, chain) -> chain.doFilter(request, response)));

    RestAssuredMockMvc.mockMvc(defaultMockMvcBuilder.build());
  }

contract =>

Contract.make {
    description "GetCustomer should return a Customer"
    request {
        method GET()
        url value(consumer(regex('/producer-service/v1/customer/ID-\\d*-\\d*')), producer("/producer-service/customer/ID-132456-9876"))
    }
    response {
        status OK()
        body(
              id: "ID-132456-9876", name: "exampleName"               
        )
        headers {
            contentType(applicationJson())
        }
    }
}

wiremocks mapping are properly generated (omitted for brevity)

Generated ContractTest =>

public class ContractVerifierTest extends BaseContractTest {

    @Test
    public void validate_shouldReturnACustomer() throws Exception {
        // given:
            MockMvcRequestSpecification request = given();


        // when:
            ResponseOptions response = given().spec(request)
                    .get("/producer-service/v1/ID-132456-9876");

        // then:
            assertThat(response.statusCode()).isEqualTo(200);
            assertThat(response.header("Content-Type")).matches("application/json.*");

        // and:
            DocumentContext parsedJson = JsonPath.parse(response.getBody().asString());
            assertThatJson(parsedJson).field("['id']").isEqualTo("ID-132456-9876");
            assertThatJson(parsedJson).field("['name']").isEqualTo("exampleName");
    }
}

when the test runs, it fails with this error:

validate_shouldReturnACustomer  Time elapsed: 0.731 s  <<< FAILURE!
java.lang.AssertionError: 

Expecting actual not to be null
    at com.example.contracts.ContractVerifierTest.validate_shouldReturnACustomer(ContractVerifierTest.java:31)

When I look up the corresponding error line, it fails on =>

assertThat(response.header("Content-Type")).matches("application/json.*");

I am a bit clueless at this point.

I tried to use the MockStandaloneApp tied to the controller (as per the link to baeldung) but that did not help.

Note that the service returns a Mono<Customer> not an actual Customer, if that changes anything.