diff --git a/src/core/ipv4/icmp.c b/src/core/ipv4/icmp.c
index 25d2584c..93a5538e 100644
--- a/src/core/ipv4/icmp.c
+++ b/src/core/ipv4/icmp.c
@@ -234,7 +234,7 @@ icmp_input(struct pbuf *p, struct netif *inp)
       ret = ip4_output_if(p, src, IP_HDRINCL,
                    ICMP_TTL, 0, IP_PROTO_ICMP, inp);
       if (ret != ERR_OK) {
-        LWIP_DEBUGF(ICMP_DEBUG, ("icmp_input: ip_output_if returned an error: %c.\n", ret));
+        LWIP_DEBUGF(ICMP_DEBUG, ("icmp_input: ip_output_if returned an error: %s\n", lwip_strerr(ret)));
       }
     }
     break;
diff --git a/src/core/ipv4/ip4.c b/src/core/ipv4/ip4.c
index e968436a..fd052c63 100644
--- a/src/core/ipv4/ip4.c
+++ b/src/core/ipv4/ip4.c
@@ -953,7 +953,7 @@ err_t ip4_output_if_opt_src(struct pbuf *p, const ip4_addr_t *src, const ip4_add
   }
 #endif /* IP_FRAG */
 
-  LWIP_DEBUGF(IP_DEBUG, ("netif->output()"));
+  LWIP_DEBUGF(IP_DEBUG, ("ip4_output_if: call netif->output()\n"));
   return netif->output(netif, p, dest);
 }