diff --git a/sys/netinet/ip_icmp.c b/sys/netinet/ip_icmp.c
--- a/sys/netinet/ip_icmp.c
+++ b/sys/netinet/ip_icmp.c
@@ -586,6 +586,8 @@
                 *   n is at least 8, but might be larger based on
                 *   ICMP_ADVLENPREF. See its definition in ip_icmp.h.
                 */
+               _Static_assert(UCHAR_MAX < IPPROTO_MAX,
+                       "ip_p is u_char; ip_ctlprotox covers all ip_p values");
                if (ip_ctlprotox[icp->icmp_ip.ip_p] != NULL)
                        ip_ctlprotox[icp->icmp_ip.ip_p](icp);
                break;