Index: sys/netinet/tcp_output.c
===================================================================
--- sys/netinet/tcp_output.c
+++ sys/netinet/tcp_output.c
@@ -1237,8 +1237,11 @@
 	if (flags & TH_SYN)
 		th->th_win = htons((u_short)
 				(min(sbspace(&so->so_rcv), TCP_MAXWIN)));
-	else
+	else {
 		th->th_win = htons((u_short)(recwin >> tp->rcv_scale));
+		if ((th->th_win == 0) && (recwin > 0))
+			th->th_win = htons(1);
+	}
 
 	/*
 	 * Adjust the RXWIN0SENT flag - indicate that we have advertised