diff --git a/tcp.c b/tcp.c
index edd3d89..47954d1 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1674,16 +1674,15 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
 		*data++ = OPT_WS;
 		*data++ = OPT_WS_LEN;
 		*data++ = conn->ws_to_tap;
-
-		th->ack = !!(flags & ACK);
 	} else {
-		th->ack = !!(flags & ACK)) ||
-			  conn->seq_ack_to_tap != prev_ack_to_tap ||
-			  !prev_wnd_to_tap;
+		if (conn->seq_ack_to_tap != prev_ack_to_tap ||
+		    !prev_wnd_to_tap)
+			flags |= ACK;
 	}
 
 	th->doff = (sizeof(*th) + optlen) / 4;
 
+	th->ack = !!(flags & ACK);
 	th->rst = !!(flags & RST);
 	th->syn = !!(flags & SYN);
 	th->fin = !!(flags & FIN);