diff --git a/sys/kern/kern_ntptime.c b/sys/kern/kern_ntptime.c
--- a/sys/kern/kern_ntptime.c
+++ b/sys/kern/kern_ntptime.c
@@ -388,7 +388,11 @@
 			time_constant = ntv->constant;
 	}
 	if (modes & MOD_TAI) {
-		if (ntv->constant > 0) /* XXX zero & negative numbers ? */
+		if (ntv->constant > 0)
+			/*
+			* Ignore zero or negative values.  The time constant must be positive
+			* to maintain stable clock discipline.
+			*/
 			time_tai = ntv->constant;
 	}
 #ifdef PPS_SYNC