diff --git a/CHANGELOG b/CHANGELOG
index cb58b380..d07be8af 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -75,6 +75,9 @@ HISTORY
 
   ++ Bugfixes:
 
+  2009-03-26 Kieran Mansley
+  * tcp_out.c: set TCP_ACK flag on keepalive and zero window probes
+
   2009-02-18 Simon Goldschmidt (Jonathan Larmour and Bill Auerbach)
   * ip_frag.c: patch #6528: the buffer used for IP_FRAG_USES_STATIC_BUF could be
     too small depending on MEM_ALIGNMENT
diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index c46e4b85..cf12a23c 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -830,8 +830,8 @@ tcp_keepalive(struct tcp_pcb *pcb)
   tcphdr->dest = htons(pcb->remote_port);
   tcphdr->seqno = htonl(pcb->snd_nxt - 1);
   tcphdr->ackno = htonl(pcb->rcv_nxt);
-  TCPH_FLAGS_SET(tcphdr, 0);
-  tcphdr->wnd = htons(pcb->rcv_ann_wnd);
+  TCPH_FLAGS_SET(tcphdr, TCP_ACK);
+  tcphdr->wnd = htons(pcb->rcv_ann_wnd + 1);
   tcphdr->urgp = 0;
   TCPH_HDRLEN_SET(tcphdr, 5);
 
@@ -913,8 +913,8 @@ tcp_zero_window_probe(struct tcp_pcb *pcb)
   tcphdr->dest = htons(pcb->remote_port);
   tcphdr->seqno = seg->tcphdr->seqno;
   tcphdr->ackno = htonl(pcb->rcv_nxt);
-  TCPH_FLAGS_SET(tcphdr, 0);
-  tcphdr->wnd = htons(pcb->rcv_ann_wnd);
+  TCPH_FLAGS_SET(tcphdr, TCP_ACK);
+  tcphdr->wnd = htons(pcb->rcv_ann_wnd + 1);
   tcphdr->urgp = 0;
   TCPH_HDRLEN_SET(tcphdr, 5);