rd_kafka_message_destroy does not free payload

[etah000]

I use cppkafka to write a consumer, and a "Message" actually called "rd_kafka_message_destroy" to free the underneath "rd_kafka_message_t". But in its subseqent called function "rd_kafka_msg_destroy", this function bypassed payload free:

if (rkm->rkm_flags & RD_KAFKA_MSG_F_FREE && rkm->rkm_payload)
		rd_free(rkm->rkm_payload);

rkm->rkm_flags is alway 0.

How can I set this flag to "RD_KAFKA_MSG_F_FREE"?