diff --git a/src/lwip_tcp/lwip_tcp_out.c b/src/lwip_tcp/lwip_tcp_out.c
index 865ffd90..a42d559c 100644
--- a/src/lwip_tcp/lwip_tcp_out.c
+++ b/src/lwip_tcp/lwip_tcp_out.c
@@ -600,8 +600,8 @@ tcp_pseudo_checksum(uint32_t src_ip, uint32_t dst_ip, uint8_t proto, uint16_t tc
     sum += (uint32_t)ntohs((uint16_t)(src_ip & 0xFFFF));
     sum += (uint32_t)ntohs((uint16_t)((dst_ip >> 16) & 0xFFFF));
     sum += (uint32_t)ntohs((uint16_t)(dst_ip & 0xFFFF));
-    sum += htons((uint16_t)proto);
-    sum += htons(tcp_len);
+    sum += (uint32_t)(uint16_t)proto;
+    sum += (uint32_t)tcp_len;
 
     const struct pbuf *q;
     for (q = p; q != NULL; q = q->next) {
@@ -655,8 +655,8 @@ tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb)
     seg->p->payload = seg->tcphdr;
 
     seg->tcphdr->chksum = 0;
-    seg->tcphdr->chksum = tcp_pseudo_checksum(pcb->local_ip, pcb->remote_ip,
-                                              IP_PROTO_TCP, seg->p->tot_len, seg->p);
+    seg->tcphdr->chksum = htons(tcp_pseudo_checksum(pcb->local_ip, pcb->remote_ip,
+                                               IP_PROTO_TCP, seg->p->tot_len, seg->p));
 
     if (pbuf_header(seg->p, (int16_t)sizeof(struct ip_hdr)) == 0) {
         iph = (struct ip_hdr *)seg->p->payload;
@@ -669,7 +669,7 @@ tcp_output_segment(struct tcp_seg *seg, struct tcp_pcb *pcb)
         iph->proto = IP_PROTO_TCP;
         iph->src = pcb->local_ip;
         iph->dst = pcb->remote_ip;
-        iph->chksum = tcp_ip_checksum(iph);
+        iph->chksum = htons(tcp_ip_checksum(iph));
         err = pcb->ctx->output(pcb->ctx->output_arg, seg->p, pcb->local_ip, pcb->remote_ip);
         pbuf_header(seg->p, -(int16_t)sizeof(struct ip_hdr));
     } else {
@@ -855,8 +855,8 @@ tcp_output_control_segment(const struct tcp_pcb *pcb, struct pbuf *p,
 
     {
         struct tcp_hdr *tcphdr = (struct tcp_hdr *)p->payload;
-        tcphdr->chksum = tcp_pseudo_checksum(src_ip, dst_ip,
-                                             IP_PROTO_TCP, p->tot_len, p);
+        tcphdr->chksum = htons(tcp_pseudo_checksum(src_ip, dst_ip,
+                                             IP_PROTO_TCP, p->tot_len, p));
     }
 
     if (pbuf_header(p, (int16_t)sizeof(struct ip_hdr)) == 0) {
@@ -876,7 +876,7 @@ tcp_output_control_segment(const struct tcp_pcb *pcb, struct pbuf *p,
         }
         iph->src = src_ip;
         iph->dst = dst_ip;
-        iph->chksum = tcp_ip_checksum(iph);
+        iph->chksum = htons(tcp_ip_checksum(iph));
         if (pcb != NULL) {
             err = pcb->ctx->output(pcb->ctx->output_arg, p, src_ip, dst_ip);
         } else {