diff --git a/sys/netinet/tcp_ecn.c b/sys/netinet/tcp_ecn.c
--- a/sys/netinet/tcp_ecn.c
+++ b/sys/netinet/tcp_ecn.c
@@ -315,7 +315,7 @@
 			}
 		} else {
 			/* RFC3168 ECN handling */
-			if (thflags & TH_ECE)
+			if ((thflags & (TH_SYN | TH_ECE)) == TH_ECE)
 				delta_ace = 1;
 			if (thflags & TH_CWR) {
 				tp->t_flags2 &= ~TF2_ECN_SND_ECE;