diff --git a/src/core/tcp_in.c b/src/core/tcp_in.c
index 4f68a92a..e6fb13a8 100644
--- a/src/core/tcp_in.c
+++ b/src/core/tcp_in.c
@@ -513,6 +513,7 @@ tcp_process(struct tcp_pcb *pcb)
       pcb->snd_wl1 = seqno - 1; /* initialise to seqno - 1 to force window update */
       pcb->state = ESTABLISHED;
       pcb->cwnd = pcb->mss;
+      LWIP_ASSERT("pcb->snd_queuelen > 0", (pcb->snd_queuelen > 0));
       --pcb->snd_queuelen;
       LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_process: SYN-SENT --queuelen %"U16_F"\n", (u16_t)pcb->snd_queuelen));
       rseg = pcb->unacked;
@@ -790,6 +791,7 @@ tcp_receive(struct tcp_pcb *pcb)
         pcb->unacked = pcb->unacked->next;
 
         LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_receive: queuelen %"U16_F" ... ", (u16_t)pcb->snd_queuelen));
+        LWIP_ASSERT("pcb->snd_queuelen >= pbuf_clen(next->p)", (pcb->snd_queuelen >= pbuf_clen(next->p)));
         pcb->snd_queuelen -= pbuf_clen(next->p);
         tcp_seg_free(next);
 
@@ -828,6 +830,7 @@ tcp_receive(struct tcp_pcb *pcb)
       next = pcb->unsent;
       pcb->unsent = pcb->unsent->next;
       LWIP_DEBUGF(TCP_QLEN_DEBUG, ("tcp_receive: queuelen %"U16_F" ... ", (u16_t)pcb->snd_queuelen));
+      LWIP_ASSERT("pcb->snd_queuelen >= pbuf_clen(next->p)", (pcb->snd_queuelen >= pbuf_clen(next->p)));
       pcb->snd_queuelen -= pbuf_clen(next->p);
       tcp_seg_free(next);
       LWIP_DEBUGF(TCP_QLEN_DEBUG, ("%"U16_F" (after freeing unsent)\n", (u16_t)pcb->snd_queuelen));
diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index 98e8f4f2..8daa3d6f 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -133,7 +133,7 @@ tcp_enqueue(struct tcp_pcb *pcb, void *arg, u16_t len,
   u32_t seqno;
   u16_t left, seglen;
   void *ptr;
-  u8_t queuelen;
+  u16_t queuelen;
 
   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));
@@ -158,8 +158,8 @@ tcp_enqueue(struct tcp_pcb *pcb, void *arg, u16_t len,
   /* If total number of pbufs on the unsent/unacked queues exceeds the
    * configured maximum, return an error */
   queuelen = pcb->snd_queuelen;
-  /* check for configured max queuelen and possible overflow of u8_t */
-  if ((queuelen >= TCP_SND_QUEUELEN) || (queuelen > 253)) {
+  /* check for configured max queuelen and possible overflow */
+  if ((queuelen >= TCP_SND_QUEUELEN) || (queuelen > TCP_SNDQUEUELEN_OVERFLOW)) {
     LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 3, ("tcp_enqueue: too long queue %"U16_F" (max %"U16_F")\n", queuelen, TCP_SND_QUEUELEN));
     TCP_STATS_INC(tcp.memerr);
     return ERR_MEM;
@@ -261,9 +261,8 @@ tcp_enqueue(struct tcp_pcb *pcb, void *arg, u16_t len,
     }
 
     /* Now that there are more segments queued, we check again if the
-    length of the queue exceeds the configured maximum. */
-    /* check for configured max queuelen and possible overflow of u8_t */
-    if ((queuelen > TCP_SND_QUEUELEN) || (queuelen > 253)) {
+    length of the queue exceeds the configured maximum or overflows. */
+    if ((queuelen > TCP_SND_QUEUELEN) || (queuelen > TCP_SNDQUEUELEN_OVERFLOW)) {
       LWIP_DEBUGF(TCP_OUTPUT_DEBUG | 2, ("tcp_enqueue: queue too long %"U16_F" (%"U16_F")\n", queuelen, TCP_SND_QUEUELEN));
       goto memerr;
     }
diff --git a/src/include/lwip/tcp.h b/src/include/lwip/tcp.h
index 668800ad..c3724ba9 100644
--- a/src/include/lwip/tcp.h
+++ b/src/include/lwip/tcp.h
@@ -118,8 +118,8 @@ void             tcp_rexmit_rto  (struct tcp_pcb *pcb);
  * unacknowledged.
  */
 #define tcp_output_nagle(tpcb) ((((tpcb)->unacked == NULL) || \
-                                 ((tpcb)->flags & TF_NODELAY) || \
-                                 (((tpcb)->unsent != NULL) && ((tpcb)->unsent->next != NULL))) ? \
+                            ((tpcb)->flags & TF_NODELAY) || \
+                            (((tpcb)->unsent != NULL) && ((tpcb)->unsent->next != NULL))) ? \
                                 tcp_output(tpcb) : ERR_OK)
 
 
@@ -302,7 +302,8 @@ struct tcp_pcb {
   u16_t acked;
   
   u16_t snd_buf;   /* Available buffer space for sending (in bytes). */
-  u8_t snd_queuelen; /* Available buffer space for sending (in tcp_segs). */
+#define TCP_SNDQUEUELEN_OVERFLOW (0xffff-3)
+  u16_t snd_queuelen; /* Available buffer space for sending (in tcp_segs). */
   
   
   /* These are ordered by sequence number: */