diff --git a/src/core/tcp_in.c b/src/core/tcp_in.c
index b0420e90..5db66839 100644
--- a/src/core/tcp_in.c
+++ b/src/core/tcp_in.c
@@ -408,6 +408,7 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
     LWIP_DEBUGF(TCP_DEBUG, ("TCP connection request %"U16_F" -> %"U16_F".\n", tcphdr->src, tcphdr->dest));
 #if TCP_LISTEN_BACKLOG
     if (pcb->accepts_pending >= pcb->backlog) {
+      LWIP_DEBUGF(TCP_DEBUG, ("tcp_listen_input: listen backlog exceeded for port %"U16_F"\n", tcphdr->dest));
       return ERR_ABRT;
     }
 #endif /* TCP_LISTEN_BACKLOG */