diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index cc982e32170..8ff015b90f8 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -854,7 +854,7 @@ tcp_packets_this_ack(struct tcpcb *tp, tcp_seq ack)
 #define	TF2_CANNOT_DO_ECN	0x00080000 /* The stack does not do ECN */
 #define	TF2_PROC_SACK_PROHIBIT	0x00100000 /* Due to small MSS size do not process sack's */
 #define	TF2_IPSEC_TSO		0x00200000 /* IPSEC + TSO supported */
-#define	TF2_NO_ISS_CHECK	0x00200000 /* Don't check SEG.ACK against ISS */
+#define	TF2_NO_ISS_CHECK	0x00400000 /* Don't check SEG.ACK against ISS */
 
 /*
  * Structure to hold TCP options that are only used during segment