diff --git a/src/netif/etharp.c b/src/netif/etharp.c
index 7765046d..9c0e749c 100644
--- a/src/netif/etharp.c
+++ b/src/netif/etharp.c
@@ -477,7 +477,7 @@ etharp_arp_input(struct netif *netif, struct eth_addr *ethaddr, struct pbuf *p)
   
   /* drop short ARP packets */
   if (p->tot_len < sizeof(struct etharp_hdr)) {
-    LWIP_DEBUGF(ETHARP_DEBUG | DBG_TRACE | 1, ("etharp_arp_input: packet dropped, too short (%"S16_F"/%"S16_F")\n", p->tot_len, sizeof(struct etharp_hdr)));
+    LWIP_DEBUGF(ETHARP_DEBUG | DBG_TRACE | 1, ("etharp_arp_input: packet dropped, too short (%"S16_F"/%"S16_F")\n", p->tot_len, (s16_t)sizeof(struct etharp_hdr)));
     pbuf_free(p);
     return;
   }