diff --git a/src/api/api_lib.c b/src/api/api_lib.c
index dc0cd8d6..620e5113 100644
--- a/src/api/api_lib.c
+++ b/src/api/api_lib.c
@@ -572,7 +572,7 @@ netconn_recv_data(struct netconn *conn, void **new_buf, u8_t apiflags)
 
 #if LWIP_TCP
 static err_t
-netconn_tcp_recvd_msg(struct netconn *conn, u32_t len, struct api_msg* msg)
+netconn_tcp_recvd_msg(struct netconn *conn, size_t len, struct api_msg* msg)
 {
   LWIP_ERROR("netconn_recv_tcp_pbuf: invalid conn", (conn != NULL) &&
              NETCONNTYPE_GROUP(netconn_type(conn)) == NETCONN_TCP, return ERR_ARG;);
@@ -584,7 +584,7 @@ netconn_tcp_recvd_msg(struct netconn *conn, u32_t len, struct api_msg* msg)
 }
 
 err_t
-netconn_tcp_recvd(struct netconn *conn, u32_t len)
+netconn_tcp_recvd(struct netconn *conn, size_t len)
 {
   err_t err;
   API_MSG_VAR_DECLARE(msg);
@@ -918,8 +918,8 @@ netconn_write_vectors_partly(struct netconn *conn, struct netvector *vectors, u1
     return ERR_OK;
   } else if (size > INT_MAX) {
     /* this is required by the socket layer (cannot send full size_t range) */
-    return ERR_VAL;
-  }
+      return ERR_VAL;
+    }
 
   API_MSG_VAR_ALLOC(msg);
   /* non-blocking write sends as much  */
diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index eff424a1..d79c37d4 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -1495,7 +1495,7 @@ lwip_netconn_do_recv(void *m)
   msg->err = ERR_OK;
   if (msg->conn->pcb.tcp != NULL) {
     if (NETCONNTYPE_GROUP(msg->conn->type) == NETCONN_TCP) {
-      u32_t remaining = msg->msg.r.len;
+      size_t remaining = msg->msg.r.len;
       do {
         u16_t recved = (remaining > 0xffff) ? 0xffff : (u16_t)remaining;
         tcp_recved(msg->conn->pcb.tcp, recved);
diff --git a/src/include/lwip/api.h b/src/include/lwip/api.h
index 901d025d..4c6bea1b 100644
--- a/src/include/lwip/api.h
+++ b/src/include/lwip/api.h
@@ -319,7 +319,7 @@ err_t   netconn_recv_udp_raw_netbuf(struct netconn *conn, struct netbuf **new_bu
 err_t   netconn_recv_udp_raw_netbuf_flags(struct netconn *conn, struct netbuf **new_buf, u8_t apiflags);
 err_t   netconn_recv_tcp_pbuf(struct netconn *conn, struct pbuf **new_buf);
 err_t   netconn_recv_tcp_pbuf_flags(struct netconn *conn, struct pbuf **new_buf, u8_t apiflags);
-err_t   netconn_tcp_recvd(struct netconn *conn, u32_t len);
+err_t   netconn_tcp_recvd(struct netconn *conn, size_t len);
 err_t   netconn_sendto(struct netconn *conn, struct netbuf *buf,
                              const ip_addr_t *addr, u16_t port);
 err_t   netconn_send(struct netconn *conn, struct netbuf *buf);
diff --git a/src/include/lwip/priv/api_msg.h b/src/include/lwip/priv/api_msg.h
index 9005e24b..c64bbb65 100644
--- a/src/include/lwip/priv/api_msg.h
+++ b/src/include/lwip/priv/api_msg.h
@@ -120,7 +120,7 @@ struct api_msg {
     } w;
     /** used for lwip_netconn_do_recv */
     struct {
-      u32_t len;
+      size_t len;
     } r;
 #if LWIP_TCP
     /** used for lwip_netconn_do_close (/shutdown) */