Contract testing messages in a Flux in Spring Cloud Stream gives IllegalArgumentException Message must not be null

I'm trying to contract test a Flux of events emitted from an endpoint using Spring Cloud Stream and Spring Cloud Contract. However, I'm getting an IllegalArgumentException with detailMessage Message must not be null when running my test. Or in other words I'm not receiving any messages on my message receiver. My spring cloud version is Hoxton. I've added the following dependencies in my pom

    <dependency>
		<groupId>org.springframework.cloud</groupId>
		<artifactId>spring-cloud-stream-binder-kafka</artifactId>
	</dependency>
    <!--TEST -->
	<dependency>
		<groupId>org.springframework.cloud</groupId>
		<artifactId>spring-cloud-stream-test-support</artifactId>
		<scope>test</scope>
	</dependency>
	<dependency>
		<groupId>org.springframework.cloud</groupId>
		<artifactId>spring-cloud-starter-contract-verifier</artifactId>
		<scope>test</scope>
	</dependency>
	<dependency>
		<groupId>org.springframework.cloud</groupId>
		<artifactId>spring-cloud-stream</artifactId>
		<type>test-jar</type>
		<scope>test</scope>
		<classifier>test-binder</classifier>
	</dependency>

I've configured a new binding following the samples here. Is working properly when I run the app and I send a curl -X POST localhost:8080 request.

application.properties

stubrunner.stream.enabled=true
spring.cloud.stream.bindings.uppercase-in-0.destination=test
spring.cloud.stream.bindings.uppercase-in-0.group=testGroup
spring.cloud.stream.bindings.uppercase-out-0.destination=hood
spring.cloud.stream.bindings.consume-in-0.destination=hood
spring.cloud.stream.bindings.consume-out-0.destination=downtown
spring.cloud.stream.bindings.supplier-out-0.destination=test
spring.cloud.function.definition=uppercase;consume;supplier

Controller

 @PostMapping(value = "/")
 public void handlePost() {
    Faker faker = new Faker();
    Message<String> message = MessageBuilder.withPayload(faker.chuckNorris().fact()).build();
    //    supplier.get();
    EventSupplier.processor.onNext(message);
 }

Bean definition

@Configuration
public class EventSupplier {
  public static final EmitterProcessor<Message<String>> processor = EmitterProcessor.create();

  @Bean
  //  public Supplier<String> supplier() {
  public Supplier<Flux<Message<String>>> supplier() {
    //    return () -> "";
    return () -> processor;
  }

}

Contracts Base test

@ActiveProfiles("test")
@SpringBootTest
@RunWith(SpringRunner.class)
@AutoConfigureMessageVerifier
@EnableBinding(KafkaStreamerApplication.class)
public abstract class KafkaStreamerApplicationTests {

  @Autowired
  private DummyController dummyController;

  public void supply() {
     dummyController.handlePost();
  }
}

Contract

org.springframework.cloud.contract.spec.Contract.make {
  label 'some_label'
  input {
   triggeredBy("supply()")
  }
  outputMessage {
    sentTo('supplier-out-0')
    body(
            anyNonBlankString()
            )
    headers {
        messagingContentType(applicationJson())
    }
  }
 }

I've tried creating contracts for function and consumer and both worked. I've also tried using a supplier of type String instead of Flux<Message<String>> and worked too. You can look at the commented lines at the code above.

While debugging I've seen that in StreamMessageCollectorMessageReceiver I'm getting a DirectWithAttributesChannel instance when retrieving the MessageChannel bean at receive("supplier-out-0", 5, "SECONDS") method. Also while looking into the documentation I've found a MessageChannel implementation that, to me, looks like a candidate to be the one should have been instantiated FluxMessageChannel.

My guess is that spring cloud contract is waiting for a message to be sent into the queue directly(DirectWithAttributesChannel) but since the flux is already in the queue and I'm just updating the flux content, it means that not message is pushed into the queue(FluxMessageChannel?) so no message is received(null) and that's why I'm getting an IllegalArgumentException with details Message must not be null error when running my tests.

Is there something missing in my configuration? Can this scenario be tested by contracts? Is a bad approach to test this by using contracts?