diff --git a/src/core/dhcp.c b/src/core/dhcp.c
index 4259c523..e944bd32 100644
--- a/src/core/dhcp.c
+++ b/src/core/dhcp.c
@@ -1356,7 +1356,9 @@ static void dhcp_delete_request(struct netif *netif)
   LWIP_ERROR("dhcp_create_request: dhcp == NULL", (dhcp == NULL), return;);
   LWIP_ASSERT("dhcp_free_msg: dhcp->p_out != NULL", dhcp->p_out != NULL);
   LWIP_ASSERT("dhcp_free_msg: dhcp->msg_out != NULL", dhcp->msg_out != NULL);
-  pbuf_free(dhcp->p_out);
+  if (dhcp->p_out != NULL) {
+    pbuf_free(dhcp->p_out);
+  }
   dhcp->p_out = NULL;
   dhcp->msg_out = NULL;
 }
diff --git a/src/core/pbuf.c b/src/core/pbuf.c
index fef4a7b1..bbc98460 100644
--- a/src/core/pbuf.c
+++ b/src/core/pbuf.c
@@ -194,7 +194,7 @@ pbuf_alloc(pbuf_layer l, u16_t length, pbuf_flag flag)
       /* make previous pbuf point to this pbuf */
       r->next = q;
       /* set total length of this pbuf and next in chain */
-      LWIP_ASSERT("rem_len < max_u16_t",rem_len < 0xffff);
+      LWIP_ASSERT("rem_len < max_u16_t", rem_len < 0xffff);
       q->tot_len = (u16_t)rem_len;
       /* this pbuf length is pool size, unless smaller sized tail */
       q->len = (rem_len > PBUF_POOL_BUFSIZE_ALIGNED) ? PBUF_POOL_BUFSIZE_ALIGNED : (u16_t)rem_len;
@@ -302,7 +302,7 @@ pbuf_realloc(struct pbuf *p, u16_t new_len)
     /* decrease remaining length by pbuf length */
     rem_len -= q->len;
     /* decrease total length indicator */
-    LWIP_ASSERT("grow < max_u16_t",grow < 0xffff);
+    LWIP_ASSERT("grow < max_u16_t", grow < 0xffff);
     q->tot_len += (u16_t)grow;
     /* proceed to next pbuf in chain */
     q = q->next;
@@ -357,15 +357,17 @@ pbuf_header(struct pbuf *p, s16_t header_size_increment)
   u16_t increment_magnitude;
 
   LWIP_ASSERT("p != NULL", p != NULL);
-  if ((header_size_increment == 0) || (p == NULL)) return 0;
+  if ((header_size_increment == 0) || (p == NULL))
+    return 0;
  
-  if (header_size_increment < 0){
-    increment_magnitude = -header_size_increment;
+  increment_magnitude = header_size_increment;
+  if (increment_magnitude < 0){
+    increment_magnitude = -increment_magnitude;
     /* Check that we aren't going to move off the end of the pbuf */
-    LWIP_ASSERT("increment_magnitude <= p->len", increment_magnitude <= p->len);
+    LWIP_ERROR("increment_magnitude > p->len", (increment_magnitude > p->len), return 1;);
   } else {
-    increment_magnitude = header_size_increment;
-#if 0 /* Can't assert these as some callers speculatively call
+#if 0
+    /* Can't assert these as some callers speculatively call
          pbuf_header() to see if it's OK.  Will return 1 below instead. */
     /* Check that we've got the correct type of pbuf to work with */
     LWIP_ASSERT("p->flags == PBUF_FLAG_RAM || p->flags == PBUF_FLAG_POOL", 
@@ -415,7 +417,7 @@ pbuf_header(struct pbuf *p, s16_t header_size_increment)
   p->len += header_size_increment;
   p->tot_len += header_size_increment;
 
-  LWIP_DEBUGF( PBUF_DEBUG, ("pbuf_header: old %p new %p (%"S16_F")\n",
+  LWIP_DEBUGF(PBUF_DEBUG, ("pbuf_header: old %p new %p (%"S16_F")\n",
     (void *)payload, (void *)p->payload, header_size_increment));
 
   return 0;
@@ -461,9 +463,9 @@ pbuf_free(struct pbuf *p)
   struct pbuf *q;
   u8_t count;
 
-  LWIP_ASSERT("p != NULL", p != NULL);
-  /* if assertions are disabled, proceed with debug output */
   if (p == NULL) {
+    LWIP_ASSERT("p != NULL", p != NULL);
+    /* if assertions are disabled, proceed with debug output */
     LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 2, ("pbuf_free(p == NULL) was called.\n"));
     return 0;
   }
@@ -575,9 +577,8 @@ pbuf_cat(struct pbuf *h, struct pbuf *t)
 {
   struct pbuf *p;
 
-  LWIP_ASSERT("h != NULL (programmer violates API)", h != NULL);
-  LWIP_ASSERT("t != NULL (programmer violates API)", t != NULL);
-  if ((h == NULL) || (t == NULL)) return;
+  LWIP_ERROR("(h == NULL) || (t == NULL) (programmer violates API)",
+             ((h == NULL) || (t == NULL)), return;);
 
   /* proceed to last pbuf of chain */
   for (p = h; p->next != NULL; p = p->next) {
@@ -688,12 +689,8 @@ pbuf_copy(struct pbuf *p_to, struct pbuf *p_from)
   LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 3, ("pbuf_copy(%p, %p)\n", p_to, p_from));
 
   /* is the target big enough to hold the source? */
-  if ((p_to == NULL) || (p_from == NULL) || (p_to->tot_len < p_from->tot_len)) {
-    LWIP_ASSERT("pbuf_copy: p_to != NULL\n", p_to != NULL);
-    LWIP_ASSERT("pbuf_copy: p_from != NULL\n", p_from != NULL);
-    LWIP_ASSERT("pbuf_copy: p_to->tot_len >= p_from->tot_len\n", p_to->tot_len >= p_from->tot_len);
-    return ERR_ARG;
-  }
+  LWIP_ERROR("pbuf_copy: target not big enough to hold source", ((p_to == NULL) ||
+             (p_from == NULL) || (p_to->tot_len < p_from->tot_len)), return ERR_ARG;);
 #ifdef LWIP_DEBUG  
   shouldbe = p_from->tot_len;
 #endif
@@ -731,19 +728,13 @@ pbuf_copy(struct pbuf *p_to, struct pbuf *p_from)
 
     if((p_from != NULL) && (p_from->len == p_from->tot_len)) {
       /* don't copy more than one packet! */
-      if (p_from->next != NULL) {
-        LWIP_ASSERT("pbuf_copy() does not allow packet queues!\n",
-                    p_from->next == NULL);
-        return ERR_VAL;
-      }
+      LWIP_ERROR("pbuf_copy() does not allow packet queues!\n",
+                 (p_from->next != NULL), return ERR_VAL;);
     }
     if((p_to != NULL) && (p_to->len == p_to->tot_len)) {
       /* don't copy more than one packet! */
-      if (p_to->next != NULL) {
-        LWIP_ASSERT("pbuf_copy() does not allow packet queues!\n",
-                    p_to->next == NULL);
-        return ERR_VAL;
-      }
+      LWIP_ERROR("pbuf_copy() does not allow packet queues!\n",
+                  (p_to->next != NULL), return ERR_VAL;);
     }
   } while (p_from);
   LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 1, ("pbuf_copy: end of chain reached.\n"));
diff --git a/src/core/sys.c b/src/core/sys.c
index a12c8d3b..98ea7c05 100644
--- a/src/core/sys.c
+++ b/src/core/sys.c
@@ -198,9 +198,10 @@ sys_timeout(u32_t msecs, sys_timeout_handler h, void *arg)
   LWIP_DEBUGF(SYS_DEBUG, ("sys_timeout: %p msecs=%"U32_F" h=%p arg=%p\n",
     (void *)timeout, msecs, (void *)h, (void *)arg));
 
-  LWIP_ASSERT("sys_timeout: timeouts != NULL", timeouts != NULL);
-  if (timeouts == NULL)
+  if (timeouts == NULL) {
+    LWIP_ASSERT("sys_timeout: timeouts != NULL", timeouts != NULL);
     return;
+  }
 
   if (timeouts->next == NULL) {
     timeouts->next = timeout;
@@ -244,11 +245,13 @@ sys_untimeout(sys_timeout_handler h, void *arg)
 
   timeouts = sys_arch_timeouts();
 
-  LWIP_ASSERT("sys_untimeout: timeouts != NULL", timeouts != NULL);
-  if (timeouts == NULL)
+  if (timeouts == NULL) {
+    LWIP_ASSERT("sys_untimeout: timeouts != NULL", timeouts != NULL);
     return;
-  if (timeouts->next == NULL)
+  }
+  if (timeouts->next == NULL) {
     return;
+  }
 
   for (t = timeouts->next, prev_t = NULL; t != NULL; prev_t = t, t = t->next) {
     if ((t->h == h) && (t->arg == arg)) {
diff --git a/src/core/tcp.c b/src/core/tcp.c
index f0540cfc..8efcd8f1 100644
--- a/src/core/tcp.c
+++ b/src/core/tcp.c
@@ -654,7 +654,7 @@ tcp_slowtmr(void)
       tcp_pcb_purge(pcb);      
       /* Remove PCB from tcp_active_pcbs list. */
       if (prev != NULL) {
-  LWIP_ASSERT("tcp_slowtmr: middle tcp != tcp_active_pcbs", pcb != tcp_active_pcbs);
+        LWIP_ASSERT("tcp_slowtmr: middle tcp != tcp_active_pcbs", pcb != tcp_active_pcbs);
         prev->next = pcb->next;
       } else {
         /* This PCB was the first. */
@@ -705,7 +705,7 @@ tcp_slowtmr(void)
       tcp_pcb_purge(pcb);      
       /* Remove PCB from tcp_tw_pcbs list. */
       if (prev != NULL) {
-  LWIP_ASSERT("tcp_slowtmr: middle tcp != tcp_tw_pcbs", pcb != tcp_tw_pcbs);
+        LWIP_ASSERT("tcp_slowtmr: middle tcp != tcp_tw_pcbs", pcb != tcp_tw_pcbs);
         prev->next = pcb->next;
       } else {
         /* This PCB was the first. */
diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index 3de9a294..257a00b8 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -137,10 +137,10 @@ tcp_enqueue(struct tcp_pcb *pcb, void *arg, u16_t len,
 
   LWIP_DEBUGF(TCP_OUTPUT_DEBUG, ("tcp_enqueue(pcb=%p, arg=%p, len=%"U16_F", flags=%"X16_F", copy=%"U16_F")\n",
     (void *)pcb, arg, len, (u16_t)flags, (u16_t)copy));
-  LWIP_ASSERT("tcp_enqueue: len == 0 || optlen == 0 (programmer violates API)",
-      len == 0 || optlen == 0);
-  LWIP_ASSERT("tcp_enqueue: arg == NULL || optdata == NULL (programmer violates API)",
-      arg == NULL || optdata == NULL);
+  LWIP_ERROR("tcp_enqueue: len != 0 && optlen != 0 (programmer violates API)",
+      (len != 0 && optlen != 0), return ERR_ARG;);
+  LWIP_ERROR("tcp_enqueue: arg != NULL && optdata != NULL (programmer violates API)",
+      (arg != NULL && optdata != NULL), return ERR_ARG;);
   /* fail on too much data */
   if (len > pcb->snd_buf) {
     LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 3, ("tcp_enqueue: too much data (len=%"U16_F" > snd_buf=%"U16_F")\n", len, pcb->snd_buf));
diff --git a/src/netif/etharp.c b/src/netif/etharp.c
index a0dfae41..25550cc3 100644
--- a/src/netif/etharp.c
+++ b/src/netif/etharp.c
@@ -403,7 +403,7 @@ update_arp_entry(struct netif *netif, struct ip_addr *ipaddr, struct eth_addr *e
   s8_t i;
   u8_t k;
   LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE | 3, ("update_arp_entry()\n"));
-  LWIP_ASSERT("netif->hwaddr_len != 0", netif->hwaddr_len != 0);
+  LWIP_ASSERT("netif->hwaddr_len == 6", netif->hwaddr_len == 6);
   LWIP_DEBUGF(ETHARP_DEBUG | LWIP_DBG_TRACE, ("update_arp_entry: %"U16_F".%"U16_F".%"U16_F".%"U16_F" - %02"X16_F":%02"X16_F":%02"X16_F":%02"X16_F":%02"X16_F":%02"X16_F"\n",
                                         ip4_addr1(ipaddr), ip4_addr2(ipaddr), ip4_addr3(ipaddr), ip4_addr4(ipaddr), 
                                         ethaddr->addr[0], ethaddr->addr[1], ethaddr->addr[2],
@@ -516,7 +516,7 @@ void
 etharp_ip_input(struct netif *netif, struct pbuf *p)
 {
   struct ethip_hdr *hdr;
-  LWIP_ASSERT("netif != NULL", netif != NULL);
+  LWIP_ERROR("netif == NULL", (netif == NULL), return;);
   /* Only insert an entry if the source IP address of the
      incoming IP packet comes from a host on the local network. */
   hdr = p->payload;
@@ -558,7 +558,7 @@ etharp_arp_input(struct netif *netif, struct eth_addr *ethaddr, struct pbuf *p)
   u8_t i;
   u8_t for_us;
 
-  LWIP_ASSERT("netif != NULL", netif != NULL);
+  LWIP_ERROR("netif == NULL", (netif == NULL), return;);
   
   /* drop short ARP packets */
   if (p->tot_len < sizeof(struct etharp_hdr)) {
@@ -858,8 +858,8 @@ etharp_query(struct netif *netif, struct ip_addr *ipaddr, struct pbuf *q)
        * PBUF_ROMs can be left as they are, since ROM must not get changed. */
       p = q;
       while(p) {
+        LWIP_ASSERT("no packet queues allowed!", (p->len == p->tot_len) || (p->next == 0));
         if(p->len == p->tot_len) {
-          LWIP_ASSERT("no packet queues allowed!", p->next == 0);
         }
         if(p->flags != PBUF_FLAG_ROM) {
           copy_needed = 1;
diff --git a/src/netif/loopif.c b/src/netif/loopif.c
index 24a52197..716577e8 100644
--- a/src/netif/loopif.c
+++ b/src/netif/loopif.c
@@ -64,10 +64,7 @@ loopif_poll(struct netif *netif)
   struct pbuf *in = NULL;
   struct loopif_private *priv = (struct loopif_private*)netif->state;
 
-  LWIP_ASSERT("priv != NULL", priv != NULL);
-  if(priv == NULL) {
-    return;
-  }
+  LWIP_ERROR("priv == NULL", (priv == NULL), return;);
 
   do {
     /* Get a packet from the list. With SYS_LIGHTWEIGHT_PROT=1, this is protected */
@@ -161,7 +158,7 @@ loopif_output(struct netif *netif, struct pbuf *p,
 
   SYS_ARCH_PROTECT(lev);
   if(priv->first != NULL) {
-    LWIP_ASSERT("if first!=NULL, last must also be != NULL", priv->last != NULL);
+    LWIP_ASSERT("if first != NULL, last must also be != NULL", priv->last != NULL);
     priv->last->next = r;
     priv->last = r;
   } else {