diff --git a/src/api/api_lib.c b/src/api/api_lib.c
index b4f08684..521d6800 100644
--- a/src/api/api_lib.c
+++ b/src/api/api_lib.c
@@ -877,13 +877,6 @@ netconn_write_vectors_partly(struct netconn *conn, struct netvector *vectors, u1
 
   LWIP_ERROR("netconn_write: invalid conn",  (conn != NULL), return ERR_ARG;);
   LWIP_ERROR("netconn_write: invalid conn->type",  (NETCONNTYPE_GROUP(conn->type)== NETCONN_TCP), return ERR_VAL;);
-  size = 0;
-  for (i = 0; i < vectorcnt; i++) {
-    size += vectors[i].len;
-  }
-  if (size == 0) {
-    return ERR_OK;
-  }
   dontblock = netconn_is_nonblocking(conn) || (apiflags & NETCONN_DONTBLOCK);
 #if LWIP_SO_SNDTIMEO
   if (conn->send_timeout != 0) {
@@ -896,6 +889,22 @@ netconn_write_vectors_partly(struct netconn *conn, struct netvector *vectors, u1
     return ERR_VAL;
   }
 
+  /* sum up the total size */
+  size = 0;
+  for (i = 0; i < vectorcnt; i++) {
+    size += vectors[i].len;
+    if (size < vectors[i].len) {
+      /* overflow */
+      return ERR_VAL;
+    }
+  }
+  if (size == 0) {
+    return ERR_OK;
+  } else if (size > INT_MAX) {
+    /* this is required by the socket layer (cannot send full size_t range) */
+    return ERR_VAL;
+  }
+
   API_MSG_VAR_ALLOC(msg);
   /* non-blocking write sends as much  */
   API_MSG_VAR_REF(msg).conn = conn;