diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index 6037ac992..b8464e29c 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -595,6 +595,9 @@ accept_function(void *arg, struct tcp_pcb *newpcb, err_t err)
     tcp_err(pcb, NULL);
     /* remove reference from to the pcb from this netconn */
     newconn->pcb.tcp = NULL;
+#if ESP_LWIP && LWIP_NETCONN_FULLDUPLEX
+    newconn->flags |= NETCONN_FLAG_MBOXINVALID;
+#endif /* ESP_LWIP && LWIP_NETCONN_FULLDUPLEX */
     /* no need to drain since we know the recvmbox is empty. */
     sys_mbox_free(&newconn->recvmbox);
     sys_mbox_set_invalid(&newconn->recvmbox);