diff --git a/sys/netgraph/l2tp/ng_l2tp.c b/sys/netgraph/l2tp/ng_l2tp.c
--- a/sys/netgraph/l2tp/ng_l2tp.c
+++ b/sys/netgraph/l2tp/ng_l2tp.c
@@ -1138,8 +1138,8 @@
 		priv->stats.recvBadAcks++;	/* ack for packet not sent */
 		return;
 	}
-	KASSERT(nack <= L2TP_MAX_XWIN,
-	    ("%s: nack=%d > %d", __func__, nack, L2TP_MAX_XWIN));
+	if (nack > L2TP_MAX_XWIN)
+		nack = L2TP_MAX_XWIN;
 
 	/* Update receive ack stats */
 	seq->rack = nr;