Index: /trunk/src/VBox/NetworkServices/NAT/pxtcp.c
===================================================================
--- /trunk/src/VBox/NetworkServices/NAT/pxtcp.c	(revision 58541)
+++ /trunk/src/VBox/NetworkServices/NAT/pxtcp.c	(revision 58542)
@@ -1043,7 +1043,9 @@
 
     /* save initial datagram in case we need to reply with ICMP */
-    pbuf_ref(p);
-    pxtcp->unsent = p;
-    pxtcp->netif = ip_current_netif();
+    if (p != NULL) {
+        pbuf_ref(p);
+        pxtcp->unsent = p;
+        pxtcp->netif = ip_current_netif();
+    }
 
     pxtcp_pcb_associate(pxtcp, newpcb);
@@ -1213,7 +1215,8 @@
 
     /* we are not going to reply with ICMP, so we can drop initial pbuf */
-    LWIP_ASSERT1(pxtcp->unsent != NULL);
-    pbuf_free(pxtcp->unsent);
-    pxtcp->unsent = NULL;
+    if (pxtcp->unsent != NULL) {
+        pbuf_free(pxtcp->unsent);
+        pxtcp->unsent = NULL;
+    }
 
     error = tcp_proxy_accept_confirm(pxtcp->pcb);