diff --git a/sys/netinet6/ip6_output.c b/sys/netinet6/ip6_output.c
index 7eea64bb63445e..9e4985cdc6cde3 100644
--- a/sys/netinet6/ip6_output.c
+++ b/sys/netinet6/ip6_output.c
@@ -798,7 +798,8 @@ ip6_output(struct mbuf *m0, struct ip6_pktopts *opt,
 			error = ENOBUFS;
 			goto bad;
 		}
-		if ((error = IPSEC_OUTPUT(ipv6, ifp, m, inp, mtu)) != 0) {
+		if ((error = IPSEC_OUTPUT(ipv6, ifp, m, inp, mtu == 0 ?
+		    ifp->if_mtu : mtu)) != 0) {
 			if (error == EINPROGRESS)
 				error = 0;
 			goto done;