diff --git a/src/netif/ppp/ppp.c b/src/netif/ppp/ppp.c
index 66b47166..5272a76f 100644
--- a/src/netif/ppp/ppp.c
+++ b/src/netif/ppp/ppp.c
@@ -809,10 +809,10 @@ void ppp_input(ppp_pcb *pcb, struct pbuf *pb) {
     pl = (u8_t*)pb->payload;
     if (pl[0] & 0x01) {
       protocol = pl[0];
-      pbuf_header(pb, -1);
+      pbuf_header(pb, -(s16_t)1);
     } else {
       protocol = (pl[0] << 8) | pl[1];
-      pbuf_header(pb, -2);
+      pbuf_header(pb, -(s16_t)2);
     }
   }
 #endif /* CCP_SUPPORT */
diff --git a/src/netif/ppp/pppoe.c b/src/netif/ppp/pppoe.c
index c3c24156..1dfa5155 100644
--- a/src/netif/ppp/pppoe.c
+++ b/src/netif/ppp/pppoe.c
@@ -666,7 +666,7 @@ pppoe_data_input(struct netif *netif, struct pbuf *pb)
 #ifdef PPPOE_TERM_UNKNOWN_SESSIONS
   MEMCPY(shost, ((struct eth_hdr *)pb->payload)->src.addr, sizeof(shost));
 #endif
-  if (pbuf_header(pb, -(int)sizeof(struct eth_hdr)) != 0) {
+  if (pbuf_header(pb, -(s16_t)sizeof(struct eth_hdr)) != 0) {
     /* bail out */
     PPPDEBUG(LOG_ERR, ("pppoe_data_input: pbuf_header failed\n"));
     LINK_STATS_INC(link.lenerr);
@@ -699,7 +699,7 @@ pppoe_data_input(struct netif *netif, struct pbuf *pb)
 
   plen = ntohs(ph->plen);
 
-  if (pbuf_header(pb, -(int)(PPPOE_HEADERLEN)) != 0) {
+  if (pbuf_header(pb, -(s16_t)(PPPOE_HEADERLEN)) != 0) {
     /* bail out */
     PPPDEBUG(LOG_ERR, ("pppoe_data_input: pbuf_header PPPOE_HEADERLEN failed\n"));
     LINK_STATS_INC(link.lenerr);
@@ -735,7 +735,7 @@ pppoe_output(struct pppoe_softc *sc, struct pbuf *pb)
   }
 
   /* make room for Ethernet header - should not fail */
-  if (pbuf_header(pb, (u16_t)(sizeof(struct eth_hdr))) != 0) {
+  if (pbuf_header(pb, (s16_t)(sizeof(struct eth_hdr))) != 0) {
     /* bail out */
     PPPDEBUG(LOG_ERR, ("pppoe: %c%c%"U16_F": pppoe_output: could not allocate room for Ethernet header\n", sc->sc_ethif->name[0], sc->sc_ethif->name[1], sc->sc_ethif->num));
     LINK_STATS_INC(link.lenerr);
@@ -1078,7 +1078,7 @@ pppoe_send_padt(struct netif *outgoing_if, u_int session, const u8_t *dest)
   }
   LWIP_ASSERT("pb->tot_len == pb->len", pb->tot_len == pb->len);
 
-  pbuf_header(pb, sizeof(struct eth_hdr));
+  pbuf_header(pb, (s16_t)sizeof(struct eth_hdr));
   ethhdr = (struct eth_hdr *)pb->payload;
   ethhdr->type = PP_HTONS(ETHTYPE_PPPOEDISC);
   MEMCPY(&ethhdr->dest.addr, dest, sizeof(ethhdr->dest.addr));
@@ -1187,7 +1187,7 @@ pppoe_xmit(struct pppoe_softc *sc, struct pbuf *pb)
   len = pb->tot_len;
 
   /* make room for PPPoE header - should not fail */
-  if (pbuf_header(pb, (u16_t)(PPPOE_HEADERLEN)) != 0) {
+  if (pbuf_header(pb, (s16_t)(PPPOE_HEADERLEN)) != 0) {
     /* bail out */
     PPPDEBUG(LOG_ERR, ("pppoe: %c%c%"U16_F": pppoe_xmit: could not allocate room for PPPoE header\n", sc->sc_ethif->name[0], sc->sc_ethif->name[1], sc->sc_ethif->num));
     LINK_STATS_INC(link.lenerr);
diff --git a/src/netif/ppp/pppol2tp.c b/src/netif/ppp/pppol2tp.c
index ed87f0e7..1514134b 100644
--- a/src/netif/ppp/pppol2tp.c
+++ b/src/netif/ppp/pppol2tp.c
@@ -443,7 +443,7 @@ static void pppol2tp_input(void *arg, struct udp_pcb *pcb, struct pbuf *p, const
   /* printf("HLEN = %d\n", hlen); */
 
   /* skip L2TP header */
-  if (pbuf_header(p, -hlen) != 0) {
+  if (pbuf_header(p, -(s16_t)hlen) != 0) {
     goto free_and_return;
   }
 
@@ -659,7 +659,7 @@ skipavp:
 nextavp:
     /* printf("AVP Found, vendor=%d, attribute=%d, len=%d\n", vendorid, attributetype, avplen); */
     /* next AVP */
-    if (pbuf_header(p, -avplen - sizeof(avpflags) - sizeof(vendorid) - sizeof(attributetype) ) != 0) {
+    if (pbuf_header(p, -(s16_t)(avplen + sizeof(avpflags) + sizeof(vendorid) + sizeof(attributetype)) ) != 0) {
       return;
     }
   }
@@ -1128,7 +1128,7 @@ static err_t pppol2tp_xmit(pppol2tp_pcb *l2tp, struct pbuf *pb) {
   }
 
   /* make room for L2TP header - should not fail */
-  if (pbuf_header(pb, PPPOL2TP_OUTPUT_DATA_HEADER_LEN) != 0) {
+  if (pbuf_header(pb, (s16_t)PPPOL2TP_OUTPUT_DATA_HEADER_LEN) != 0) {
     /* bail out */
     PPPDEBUG(LOG_ERR, ("pppol2tp: pppol2tp_pcb: could not allocate room for L2TP header\n"));
     LINK_STATS_INC(link.lenerr);
diff --git a/src/netif/ppp/vj.c b/src/netif/ppp/vj.c
index d44eee45..71fbc68d 100644
--- a/src/netif/ppp/vj.c
+++ b/src/netif/ppp/vj.c
@@ -374,7 +374,7 @@ vj_compress_tcp(struct vjcompress *comp, struct pbuf *pb)
   if (!comp->compressSlot || comp->last_xmit != cs->cs_id) {
     comp->last_xmit = cs->cs_id;
     hlen -= deltaS + 4;
-    if(pbuf_header(pb, -hlen)){
+    if (pbuf_header(pb, -(s16_t)hlen)){
       /* Can we cope with this failing?  Just assert for now */
       LWIP_ASSERT("pbuf_header failed\n", 0);
     }
@@ -383,7 +383,7 @@ vj_compress_tcp(struct vjcompress *comp, struct pbuf *pb)
     *cp++ = cs->cs_id;
   } else {
     hlen -= deltaS + 3;
-    if(pbuf_header(pb, -hlen)) {
+    if (pbuf_header(pb, -(s16_t)hlen)) {
       /* Can we cope with this failing?  Just assert for now */
       LWIP_ASSERT("pbuf_header failed\n", 0);
     }
@@ -585,7 +585,7 @@ vj_uncompress_tcp(struct pbuf **nb, struct vjcompress *comp)
   IPH_CHKSUM_SET(&cs->cs_ip,  (u_short)(~tmp));
   
   /* Remove the compressed header and prepend the uncompressed header. */
-  if(pbuf_header(n0, -((s16_t)(vjlen)))) {
+  if (pbuf_header(n0, -(s16_t)vjlen)) {
     /* Can we cope with this failing?  Just assert for now */
     LWIP_ASSERT("pbuf_header failed\n", 0);
     goto bad;
@@ -609,7 +609,7 @@ vj_uncompress_tcp(struct pbuf **nb, struct vjcompress *comp)
       goto bad;
     }
 
-    if(pbuf_header(np, -cs->cs_hlen)) {
+    if (pbuf_header(np, -(s16_t)cs->cs_hlen)) {
       /* Can we cope with this failing?  Just assert for now */
       LWIP_ASSERT("pbuf_header failed\n", 0);
       goto bad;
@@ -629,7 +629,7 @@ vj_uncompress_tcp(struct pbuf **nb, struct vjcompress *comp)
     n0 = np;
   }
 
-  if(pbuf_header(n0, cs->cs_hlen)) {
+  if (pbuf_header(n0, (s16_t)cs->cs_hlen)) {
     struct pbuf *np;
 
     LWIP_ASSERT("vj_uncompress_tcp: cs->cs_hlen <= PBUF_POOL_BUFSIZE", cs->cs_hlen <= PBUF_POOL_BUFSIZE);