diff --git a/src/core/ipv4/ip4.c b/src/core/ipv4/ip4.c
index bda49d2b..7b4dfa3d 100644
--- a/src/core/ipv4/ip4.c
+++ b/src/core/ipv4/ip4.c
@@ -1064,6 +1064,8 @@ ip4_output_if_opt_src(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *d
   if (netif->mtu && (p->tot_len > netif->mtu)) {
     return ip4_frag(p, netif, dest);
   }
+#else
+  LWIP_ERROR("ip4_output_if: Packet larger than MTU discarded", !(netif->mtu && p->tot_len > netif->mtu), return ERR_IF;);
 #endif /* IP_FRAG */
 
   LWIP_DEBUGF(IP_DEBUG, ("ip4_output_if: call netif->output()\n"));