diff --git a/src/core/snmp/asn1_enc.c b/src/core/snmp/asn1_enc.c
index 77af6b4b..8f76bd9b 100644
--- a/src/core/snmp/asn1_enc.c
+++ b/src/core/snmp/asn1_enc.c
@@ -227,7 +227,7 @@ snmp_asn1_enc_length(struct pbuf *p, u16_t ofs, u16_t length)
 
       if (length < 0x80)
       {
-        *msg_ptr = length;
+        *msg_ptr = (u8_t)length;
         return ERR_OK;
       }
       else if (length < 0x100)
@@ -246,7 +246,7 @@ snmp_asn1_enc_length(struct pbuf *p, u16_t ofs, u16_t length)
           /* next octet in same pbuf */
           msg_ptr++;
         }
-        *msg_ptr = length;
+        *msg_ptr = (u8_t)length;
         return ERR_OK;
       }
       else
@@ -276,12 +276,12 @@ snmp_asn1_enc_length(struct pbuf *p, u16_t ofs, u16_t length)
           if (i == 0)
           {
             /* least significant length octet */
-            *msg_ptr = length;
+            *msg_ptr = (u8_t)length;
           }
           else
           {
             /* most significant length octet */
-            *msg_ptr = length >> 8;
+            *msg_ptr = (u8_t)(length >> 8);
           }
         }
         return ERR_OK;
@@ -305,7 +305,7 @@ snmp_asn1_enc_length(struct pbuf *p, u16_t ofs, u16_t length)
  * @see snmp_asn1_enc_u32t_cnt()
  */
 err_t
-snmp_asn1_enc_u32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, u32_t value)
+snmp_asn1_enc_u32t(struct pbuf *p, u16_t ofs, u16_t octets_needed, u32_t value)
 {
   u16_t plen, base;
   u8_t *msg_ptr;
@@ -343,7 +343,7 @@ snmp_asn1_enc_u32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, u32_t value)
       while (octets_needed > 1)
       {
         octets_needed--;
-        *msg_ptr = value >> (octets_needed << 3);
+        *msg_ptr = (u8_t)(value >> (octets_needed << 3));
         ofs += 1;
         if (ofs >= plen)
         {
@@ -360,7 +360,7 @@ snmp_asn1_enc_u32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, u32_t value)
         }
       }
       /* (only) one least significant octet */
-      *msg_ptr = value;
+      *msg_ptr = (u8_t)value;
       return ERR_OK;
     }
     p = p->next;
@@ -381,7 +381,7 @@ snmp_asn1_enc_u32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, u32_t value)
  * @see snmp_asn1_enc_s32t_cnt()
  */
 err_t
-snmp_asn1_enc_s32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, s32_t value)
+snmp_asn1_enc_s32t(struct pbuf *p, u16_t ofs, u16_t octets_needed, s32_t value)
 {
   u16_t plen, base;
   u8_t *msg_ptr;
@@ -399,7 +399,7 @@ snmp_asn1_enc_s32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, s32_t value)
       while (octets_needed > 1)
       {
         octets_needed--;
-        *msg_ptr = value >> (octets_needed << 3);
+        *msg_ptr = (u8_t)(value >> (octets_needed << 3));
         ofs += 1;
         if (ofs >= plen)
         {
@@ -416,7 +416,7 @@ snmp_asn1_enc_s32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, s32_t value)
         }
       }
       /* (only) one least significant octet */
-      *msg_ptr = value;
+      *msg_ptr = (u8_t)value;
       return ERR_OK;
     }
     p = p->next;
@@ -460,7 +460,7 @@ snmp_asn1_enc_oid(struct pbuf *p, u16_t ofs, u8_t ident_len, s32_t *ident)
         else
         {
           /* calculate prefix */
-          *msg_ptr = (ident[0] * 40) + ident[1];
+          *msg_ptr = (u8_t)((ident[0] * 40) + ident[1]);
         }
         ofs += 1;
         if (ofs >= plen)
@@ -498,7 +498,7 @@ snmp_asn1_enc_oid(struct pbuf *p, u16_t ofs, u8_t ident_len, s32_t *ident)
         {
           u8_t code;
 
-          code = sub_id >> shift;
+          code = (u8_t)(sub_id >> shift);
           if ((code != 0) || (tail != 0))
           {
             tail = 1;
@@ -559,11 +559,13 @@ snmp_asn1_enc_oid(struct pbuf *p, u16_t ofs, u8_t ident_len, s32_t *ident)
  * @return ERR_OK if successfull, ERR_ARG if we can't (or won't) encode
  */
 err_t
-snmp_asn1_enc_raw(struct pbuf *p, u16_t ofs, u8_t raw_len, u8_t *raw)
+snmp_asn1_enc_raw(struct pbuf *p, u16_t ofs, u16_t raw_len, u8_t *raw)
 {
   u16_t plen, base;
   u8_t *msg_ptr;
 
+  LWIP_ASSERT("raw_len <= 0xffff", raw_len <= 0xffff);
+
   plen = 0;
   while (p != NULL)
   {
diff --git a/src/core/snmp/mib2.c b/src/core/snmp/mib2.c
index bc5830d6..afb3c312 100644
--- a/src/core/snmp/mib2.c
+++ b/src/core/snmp/mib2.c
@@ -898,11 +898,11 @@ static u32_t snmpinpkts = 0,
  * @param src points to source
  * @param n number of octets to copy.
  */
-void ocstrncpy(u8_t *dst, u8_t *src, u8_t n)
+static void ocstrncpy(u8_t *dst, u8_t *src, u16_t n)
 {
-  while (n > 0)
-  {
-    n--;
+  u16_t i = n;
+  while (i > 0) {
+    i--;
     *dst++ = *src++;
   }
 }
@@ -916,9 +916,9 @@ void ocstrncpy(u8_t *dst, u8_t *src, u8_t n)
  */
 void objectidncpy(s32_t *dst, s32_t *src, u8_t n)
 {
-  while(n > 0)
-  {
-    n--;
+  u8_t i = n;
+  while(i > 0) {
+    i--;
     *dst++ = *src++;
   }
 }
@@ -2129,7 +2129,8 @@ system_get_object_def(u8_t ident_len, s32_t *ident, struct obj_def *od)
     od->id_inst_len = ident_len;
     od->id_inst_ptr = ident;
 
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     LWIP_DEBUGF(SNMP_MIB_DEBUG,("get_object_def system.%"U16_F".0\n",(u16_t)id));
     switch (id)
     {
@@ -2201,7 +2202,8 @@ system_get_value(struct obj_def *od, u16_t len, void *value)
 {
   u8_t id;
 
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 1: /* sysDescr */
@@ -2240,7 +2242,8 @@ system_set_test(struct obj_def *od, u16_t len, void *value)
 
   LWIP_UNUSED_ARG(value);
   set_ok = 0;
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 4: /* sysContact */
@@ -2273,20 +2276,22 @@ system_set_value(struct obj_def *od, u16_t len, void *value)
 {
   u8_t id;
 
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid len", len <= 0xff);
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 4: /* sysContact */
       ocstrncpy(syscontact_ptr,value,len);
-      *syscontact_len_ptr = len;
+      *syscontact_len_ptr = (u8_t)len;
       break;
     case 5: /* sysName */
       ocstrncpy(sysname_ptr,value,len);
-      *sysname_len_ptr = len;
+      *sysname_len_ptr = (u8_t)len;
       break;
     case 6: /* sysLocation */
       ocstrncpy(syslocation_ptr,value,len);
-      *syslocation_len_ptr = len;
+      *syslocation_len_ptr = (u8_t)len;
       break;
   };
 }
@@ -2360,7 +2365,8 @@ ifentry_get_object_def(u8_t ident_len, s32_t *ident, struct obj_def *od)
     od->id_inst_len = ident_len;
     od->id_inst_ptr = ident;
 
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     LWIP_DEBUGF(SNMP_MIB_DEBUG,("get_object_def ifentry.%"U16_F"\n",(u16_t)id));
     switch (id)
     {
@@ -2461,7 +2467,8 @@ ifentry_get_value(struct obj_def *od, u16_t len, void *value)
   u8_t id;
 
   snmp_ifindextonetif(od->id_inst_ptr[1], &netif);
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 1: /* ifIndex */
@@ -2734,7 +2741,8 @@ atentry_get_value(struct obj_def *od, u16_t len, void *value)
 #if LWIP_ARP /** @todo implement a netif_find_addr */
   if (etharp_find_addr(netif, &ip, &ethaddr_ret, &ipaddr_ret) > -1)
   {
-    id = od->id_inst_ptr[0];
+    LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+    id = (u8_t)od->id_inst_ptr[0];
     switch (id)
     {
       case 1: /* atIfIndex */
@@ -2775,7 +2783,8 @@ ip_get_object_def(u8_t ident_len, s32_t *ident, struct obj_def *od)
     od->id_inst_len = ident_len;
     od->id_inst_ptr = ident;
 
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     LWIP_DEBUGF(SNMP_MIB_DEBUG,("get_object_def ip.%"U16_F".0\n",(u16_t)id));
     switch (id)
     {
@@ -2833,7 +2842,8 @@ ip_get_value(struct obj_def *od, u16_t len, void *value)
   u8_t id;
 
   LWIP_UNUSED_ARG(len);
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 1: /* ipForwarding */
@@ -2988,7 +2998,8 @@ ip_set_test(struct obj_def *od, u16_t len, void *value)
 
   LWIP_UNUSED_ARG(len);
   set_ok = 0;
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 1: /* ipForwarding */
@@ -3027,7 +3038,8 @@ ip_addrentry_get_object_def(u8_t ident_len, s32_t *ident, struct obj_def *od)
     od->id_inst_len = ident_len;
     od->id_inst_ptr = ident;
 
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     switch (id)
     {
       case 1: /* ipAdEntAddr */
@@ -3078,7 +3090,8 @@ ip_addrentry_get_value(struct obj_def *od, u16_t len, void *value)
 
   if (netif != NULL)
   {
-    id = od->id_inst_ptr[0];
+    LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+    id = (u8_t)od->id_inst_ptr[0];
     switch (id)
     {
       case 1: /* ipAdEntAddr */
@@ -3148,7 +3161,8 @@ ip_rteentry_get_object_def(u8_t ident_len, s32_t *ident, struct obj_def *od)
     od->id_inst_len = ident_len;
     od->id_inst_ptr = ident;
 
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     switch (id)
     {
       case 1: /* ipRouteDest */
@@ -3227,7 +3241,8 @@ ip_rteentry_get_value(struct obj_def *od, u16_t len, void *value)
   }
   if (netif != NULL)
   {
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     switch (id)
     {
       case 1: /* ipRouteDest */
@@ -3363,7 +3378,8 @@ ip_ntomentry_get_object_def(u8_t ident_len, s32_t *ident, struct obj_def *od)
     od->id_inst_len = ident_len;
     od->id_inst_ptr = ident;
 
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     switch (id)
     {
       case 1: /* ipNetToMediaIfIndex */
@@ -3419,7 +3435,8 @@ ip_ntomentry_get_value(struct obj_def *od, u16_t len, void *value)
 #if LWIP_ARP /** @todo implement a netif_find_addr */
   if (etharp_find_addr(netif, &ip, &ethaddr_ret, &ipaddr_ret) > -1)
   {
-    id = od->id_inst_ptr[0];
+    LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+    id = (u8_t)od->id_inst_ptr[0];
     switch (id)
     {
       case 1: /* ipNetToMediaIfIndex */
@@ -3485,7 +3502,8 @@ icmp_get_value(struct obj_def *od, u16_t len, void *value)
   u8_t id;
 
   LWIP_UNUSED_ARG(len);
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 1: /* icmpInMsgs */
@@ -3584,7 +3602,8 @@ tcp_get_object_def(u8_t ident_len, s32_t *ident, struct obj_def *od)
     od->id_inst_len = ident_len;
     od->id_inst_ptr = ident;
 
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     LWIP_DEBUGF(SNMP_MIB_DEBUG,("get_object_def tcp.%"U16_F".0\n",(u16_t)id));
 
     switch (id)
@@ -3639,7 +3658,8 @@ tcp_get_value(struct obj_def *od, u16_t len, void *value)
   u8_t id;
 
   LWIP_UNUSED_ARG(len);
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 1: /* tcpRtoAlgorithm, vanj(4) */
@@ -3807,7 +3827,8 @@ udp_get_value(struct obj_def *od, u16_t len, void *value)
   u8_t id;
 
   LWIP_UNUSED_ARG(len);
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
     case 1: /* udpInDatagrams */
@@ -3875,7 +3896,8 @@ udpentry_get_value(struct obj_def *od, u16_t len, void *value)
   LWIP_UNUSED_ARG(len);
   snmp_oidtoip(&od->id_inst_ptr[1], &ip);
   ip.addr = htonl(ip.addr);
-  port = od->id_inst_ptr[5];
+  LWIP_ASSERT("invalid port", (od->id_inst_ptr[5] >= 0) && (od->id_inst_ptr[5] <= 0xffff));
+  port = (u16_t)od->id_inst_ptr[5];
 
   pcb = udp_pcbs;
   while ((pcb != NULL) &&
@@ -3887,7 +3909,8 @@ udpentry_get_value(struct obj_def *od, u16_t len, void *value)
 
   if (pcb != NULL)
   {
-    id = od->id_inst_ptr[0];
+    LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+    id = (u8_t)od->id_inst_ptr[0];
     switch (id)
     {
       case 1: /* udpLocalAddress */
@@ -3919,7 +3942,8 @@ snmp_get_object_def(u8_t ident_len, s32_t *ident, struct obj_def *od)
     od->id_inst_len = ident_len;
     od->id_inst_ptr = ident;
 
-    id = ident[0];
+    LWIP_ASSERT("invalid id", (ident[0] >= 0) && (ident[0] <= 0xff));
+    id = (u8_t)ident[0];
     switch (id)
     {
       case 1: /* snmpInPkts */
@@ -3980,7 +4004,8 @@ snmp_get_value(struct obj_def *od, u16_t len, void *value)
   u8_t id;
 
   LWIP_UNUSED_ARG(len);
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   switch (id)
   {
       case 1: /* snmpInPkts */
@@ -4084,7 +4109,8 @@ snmp_set_test(struct obj_def *od, u16_t len, void *value)
 
   LWIP_UNUSED_ARG(len);
   set_ok = 0;
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   if (id == 30)
   {
     /* snmpEnableAuthenTraps */
@@ -4116,12 +4142,14 @@ snmp_set_value(struct obj_def *od, u16_t len, void *value)
   u8_t id;
 
   LWIP_UNUSED_ARG(len);
-  id = od->id_inst_ptr[0];
+  LWIP_ASSERT("invalid id", (od->id_inst_ptr[0] >= 0) && (od->id_inst_ptr[0] <= 0xff));
+  id = (u8_t)od->id_inst_ptr[0];
   if (id == 30)
   {
     /* snmpEnableAuthenTraps */
-    s32_t *sint_ptr = value;
-    *snmpenableauthentraps_ptr = *sint_ptr;
+    /* @todo @fixme: which kind of pointer is 'value'? s32_t or u8_t??? */
+    u8_t *ptr = value;
+    *snmpenableauthentraps_ptr = *ptr;
   }
 }
 
diff --git a/src/core/snmp/mib_structs.c b/src/core/snmp/mib_structs.c
index 5f488b1e..f4dc2929 100644
--- a/src/core/snmp/mib_structs.c
+++ b/src/core/snmp/mib_structs.c
@@ -95,7 +95,7 @@ void
 snmp_ifindextonetif(s32_t ifindex, struct netif **netif)
 {
   struct netif *nif = netif_list;
-  u16_t i, ifidx;
+  s32_t i, ifidx;
 
   ifidx = ifindex - 1;
   i = 0;
@@ -754,7 +754,8 @@ snmp_expand_tree(struct mib_node *node, u8_t ident_len, s32_t *ident, struct snm
 
             LWIP_DEBUGF(SNMP_MIB_DEBUG,("non-leaf node\n"));
             /* non-leaf, store right child ptr and id */
-            j = i + 1;
+            LWIP_ASSERT("i < 0xff", i < 0xff);
+            j = (u8_t)i + 1;
             while ((j < an->maxlength) && (empty_table(an->nptr[j])))
             {
               j++;
@@ -996,7 +997,8 @@ snmp_expand_tree(struct mib_node *node, u8_t ident_len, s32_t *ident, struct snm
 
             LWIP_DEBUGF(SNMP_MIB_DEBUG,("non-leaf node\n"));
             /* non-leaf, store right child ptr and id */
-            j = i + 1;
+            LWIP_ASSERT("i < 0xff", i < 0xff);
+            j = (u8_t)i + 1;
             if (j < len)
             {
               /* right node is the current external node */
diff --git a/src/core/snmp/msg_in.c b/src/core/snmp/msg_in.c
index 49aaa2f4..194b67df 100644
--- a/src/core/snmp/msg_in.c
+++ b/src/core/snmp/msg_in.c
@@ -186,7 +186,8 @@ snmp_msg_get_event(u8_t request_id, struct snmp_msg_pstat *msg_ps)
       msg_ps->vb_ptr->ident_len = 0;
 
       vb->value_type = msg_ps->ext_object_def.asn_type;
-      vb->value_len =  msg_ps->ext_object_def.v_len;
+      LWIP_ASSERT("invalid length", msg_ps->ext_object_def.v_len <= 0xff);
+      vb->value_len = (u8_t)msg_ps->ext_object_def.v_len;
       if (vb->value_len > 0)
       {
         vb->value = mem_malloc(vb->value_len);
@@ -298,7 +299,8 @@ snmp_msg_get_event(u8_t request_id, struct snmp_msg_pstat *msg_ps)
               msg_ps->vb_ptr->ident_len = 0;
 
               vb->value_type = object_def.asn_type;
-              vb->value_len = object_def.v_len;
+              LWIP_ASSERT("invalid length", object_def.v_len <= 0xff);
+              vb->value_len = (u8_t)object_def.v_len;
               if (vb->value_len > 0)
               {
                 vb->value = mem_malloc(vb->value_len);
@@ -394,9 +396,10 @@ snmp_msg_getnext_event(u8_t request_id, struct snmp_msg_pstat *msg_ps)
     /* get_value() answer */
     en = msg_ps->ext_mib_node;
 
+    LWIP_ASSERT("invalid length", msg_ps->ext_object_def.v_len <= 0xff);
     vb = snmp_varbind_alloc(&msg_ps->ext_oid,
                             msg_ps->ext_object_def.asn_type,
-                            msg_ps->ext_object_def.v_len);
+                            (u8_t)msg_ps->ext_object_def.v_len);
     if (vb != NULL)
     {
       en->get_value_a(request_id, &msg_ps->ext_object_def, vb->value_len, vb->value);
@@ -468,7 +471,8 @@ snmp_msg_getnext_event(u8_t request_id, struct snmp_msg_pstat *msg_ps)
         msg_ps->state = SNMP_MSG_INTERNAL_GET_OBJDEF;
         mn->get_object_def(1, &oid.id[oid.len - 1], &object_def);
 
-        vb = snmp_varbind_alloc(&oid, object_def.asn_type, object_def.v_len);
+        LWIP_ASSERT("invalid length", object_def.v_len <= 0xff);
+        vb = snmp_varbind_alloc(&oid, object_def.asn_type, (u8_t)object_def.v_len);
         if (vb != NULL)
         {
           msg_ps->state = SNMP_MSG_INTERNAL_GET_VALUE;
@@ -978,7 +982,7 @@ snmp_pdu_header_check(struct pbuf *p, u16_t ofs, u16_t pdu_len, u16_t *ofs_ret,
   /* add zero terminator */
   len = ((len < (SNMP_COMMUNITY_STR_LEN))?(len):(SNMP_COMMUNITY_STR_LEN));
   m_stat->community[len] = 0;
-  m_stat->com_strlen = len;
+  m_stat->com_strlen = (u8_t)len;
   if (strncmp(snmp_publiccommunity, (const char*)m_stat->community, SNMP_COMMUNITY_STR_LEN) != 0)
   {
     /** @todo: move this if we need to check more names */
@@ -1223,7 +1227,8 @@ snmp_pdu_dec_varbindlist(struct pbuf *p, u16_t ofs, u16_t *ofs_ret, struct snmp_
         break;
       case (SNMP_ASN1_UNIV | SNMP_ASN1_PRIMIT | SNMP_ASN1_OC_STR):
       case (SNMP_ASN1_APPLIC | SNMP_ASN1_PRIMIT | SNMP_ASN1_OPAQUE):
-        vb = snmp_varbind_alloc(&oid, type, len);
+        LWIP_ASSERT("invalid length", len <= 0xff);
+        vb = snmp_varbind_alloc(&oid, type, (u8_t)len);
         if (vb != NULL)
         {
           derr = snmp_asn1_dec_raw(p, ofs + 1 + len_octets, len, vb->value_len, vb->value);
diff --git a/src/include/lwip/snmp_asn1.h b/src/include/lwip/snmp_asn1.h
index 8a602881..b6996ca9 100644
--- a/src/include/lwip/snmp_asn1.h
+++ b/src/include/lwip/snmp_asn1.h
@@ -87,10 +87,10 @@ void snmp_asn1_enc_s32t_cnt(s32_t value, u16_t *octets_needed);
 void snmp_asn1_enc_oid_cnt(u8_t ident_len, s32_t *ident, u16_t *octets_needed);
 err_t snmp_asn1_enc_type(struct pbuf *p, u16_t ofs, u8_t type);
 err_t snmp_asn1_enc_length(struct pbuf *p, u16_t ofs, u16_t length);
-err_t snmp_asn1_enc_u32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, u32_t value);
-err_t snmp_asn1_enc_s32t(struct pbuf *p, u16_t ofs, u8_t octets_needed, s32_t value);
+err_t snmp_asn1_enc_u32t(struct pbuf *p, u16_t ofs, u16_t octets_needed, u32_t value);
+err_t snmp_asn1_enc_s32t(struct pbuf *p, u16_t ofs, u16_t octets_needed, s32_t value);
 err_t snmp_asn1_enc_oid(struct pbuf *p, u16_t ofs, u8_t ident_len, s32_t *ident);
-err_t snmp_asn1_enc_raw(struct pbuf *p, u16_t ofs, u8_t raw_len, u8_t *raw);
+err_t snmp_asn1_enc_raw(struct pbuf *p, u16_t ofs, u16_t raw_len, u8_t *raw);
 
 #ifdef __cplusplus
 }