diff --git a/tcp.c b/tcp.c
index 4c24367..32a08bd 100644
--- a/tcp.c
+++ b/tcp.c
@@ -787,7 +787,8 @@ static void tcp_sock_set_nodelay(int s)
  * @th:		TCP header (updated)
  * @payload:	TCP payload
  */
-void tcp_update_csum(uint32_t psum, struct tcphdr *th, struct iov_tail *payload)
+static void tcp_update_csum(uint32_t psum, struct tcphdr *th,
+			    struct iov_tail *payload)
 {
 	th->check = 0;
 	psum = csum_unfolded(th, sizeof(*th), psum);
diff --git a/tcp_internal.h b/tcp_internal.h
index 9cf31f5..6f5e054 100644
--- a/tcp_internal.h
+++ b/tcp_internal.h
@@ -166,8 +166,6 @@ void tcp_rst_do(const struct ctx *c, struct tcp_tap_conn *conn);
 
 struct tcp_info_linux;
 
-void tcp_update_csum(uint32_t psum, struct tcphdr *th,
-		     struct iov_tail *payload);
 void tcp_fill_headers(const struct tcp_tap_conn *conn,
 		      struct tap_hdr *taph,
 		      struct iphdr *ip4h, struct ipv6hdr *ip6h,