diff --git a/lib/isc/unix/socket.c b/lib/isc/unix/socket.c
index 874f4641fc..6fb88609e6 100644
--- a/lib/isc/unix/socket.c
+++ b/lib/isc/unix/socket.c
@@ -2266,6 +2266,9 @@ clear_bsdcompat(void) {
 
 static void
 use_min_mtu(isc__socket_t *sock) {
+#if !defined(IPV6_USE_MIN_MTU) && !defined(IPV6_MTU)
+	UNUSED(sock);
+#endif
 #ifdef IPV6_USE_MIN_MTU
 	/* use minimum MTU */
 	if (sock->pf == AF_INET6) {
diff --git a/lib/isc/win32/socket.c b/lib/isc/win32/socket.c
index a5053c64a4..ec0ab7610b 100644
--- a/lib/isc/win32/socket.c
+++ b/lib/isc/win32/socket.c
@@ -1432,7 +1432,7 @@ startio_send(isc_socket_t *sock, isc_socketevent_t *dev, int *nbytes,
 }
 
 static void
-use_min_mtu(isc__socket_t *sock) {
+use_min_mtu(isc_socket_t *sock) {
 #ifdef IPV6_USE_MIN_MTU
 	/* use minimum MTU */
 	if (sock->pf == AF_INET6) {
@@ -1440,6 +1440,8 @@ use_min_mtu(isc__socket_t *sock) {
 		(void)setsockopt(sock->fd, IPPROTO_IPV6, IPV6_USE_MIN_MTU,
 				(void *)&on, sizeof(on));
 	}
+#else
+	UNUSED(sock);
 #endif
 }