How to produce a Tombstone in Kafka using dotnet kafkaflow?

Question

I am trying to produce tombstone messages to a Kafka topic which is used as a source in a Flink program.

I was able to successfully produce the record using Confluent.Kafka using something like this

using var producer = new ProducerBuilder<string, Null>(conf).Build();
producer.Produce("test", new Message<string, Null>
{
    Key = key, 
});
producer.Flush();

but it doesn't work when trying to do it using KafkaFlow

//injection
services.AddKafka(kafka => kafka
    .AddCluster(cluster => cluster
        .WithBrokers(new[] { kafkaBrokers })
        .AddProducer("test"), producer => producer
            .AddMiddlewares(middlewares => middlewares
                .AddSerializer<JsonCoreSerializer>()
            )
            .WithAcks(Acks.All)
        )

//usage
producer.ProduceAsync(topic, key, null);