diff --git a/kernel-module/extmap_filter.inc.c b/kernel-module/extmap_filter.inc.c
index 4c50d3919..3743a9b94 100644
--- a/kernel-module/extmap_filter.inc.c
+++ b/kernel-module/extmap_filter.inc.c
@@ -241,6 +241,7 @@ static void apply_extmap_filter(struct sk_buff *skb, struct rtpengine_output *o,
 		else
 			add_extmap_hdr_short(skb, o, rtp);
 	}
+	rtp->header_len = rtp->payload - (unsigned char *) rtp->rtp_header;
 }
 
 
diff --git a/t/kernel-extmap-filter.c b/t/kernel-extmap-filter.c
index 6a51eea83..d2b930a63 100644
--- a/t/kernel-extmap-filter.c
+++ b/t/kernel-extmap-filter.c
@@ -35,6 +35,7 @@ struct rtp_exthdr {
 
 struct rtp_parsed {
 	struct rtp_header *rtp_header;
+	size_t header_len;
 	unsigned char *payload;
 	size_t payload_len;
 	struct rtp_exthdr *ext_hdr;