diff --git a/CHANGELOG b/CHANGELOG
index 2f3b4b32..64350e9a 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -80,6 +80,10 @@ HISTORY
 
  ++ Bugfixes:
 
+  2012-05-08: Simon Goldschmidt
+  * tcp_out.c: fixed bug: #36380 unsent_oversize mismatch in 1.4.1RC1 (this was
+    a debug-check issue only)
+
   2012-05-03: Simon Goldschmidt (patch by Sylvain Rochet)
   * ppp.c: fixed bug #36283 (PPP struct used on header size computation and
     not packed)
diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index 74c4900d..8cfb3df9 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -483,7 +483,7 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
           goto memerr;
         }
 #if TCP_OVERSIZE_DBGCHECK
-        last_unsent->oversize_left = oversize;
+        last_unsent->oversize_left += oversize;
 #endif /* TCP_OVERSIZE_DBGCHECK */
         TCP_DATA_COPY2(concat_p->payload, (u8_t*)arg + pos, seglen, &concat_chksum, &concat_chksum_swapped);
 #if TCP_CHECKSUM_ON_COPY
@@ -638,6 +638,8 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
     }
     last_unsent->len += oversize_used;
 #if TCP_OVERSIZE_DBGCHECK
+    LWIP_ASSERT("last_unsent->oversize_left >= oversize_used",
+                last_unsent->oversize_left >= oversize_used);
     last_unsent->oversize_left -= oversize_used;
 #endif /* TCP_OVERSIZE_DBGCHECK */
   }