diff --git a/sys/netinet/sctp_indata.c b/sys/netinet/sctp_indata.c
index e6713a339933..83ab59f0ff31 100644
--- a/sys/netinet/sctp_indata.c
+++ b/sys/netinet/sctp_indata.c
@@ -2245,15 +2245,19 @@ sctp_slide_mapping_arrays(struct sctp_tcb *stcb)
 	/*
 	 * Now we also need to check the mapping array in a couple of ways.
 	 * 1) Did we move the cum-ack point?
+	 * 
+	 * When you first glance at this you might think that all entries that
+	 * make up the postion of the cum-ack would be in the nr-mapping
+	 * array only.. i.e. things up to the cum-ack are always
+	 * deliverable. Thats true with one exception, when its a fragmented
+	 * message we may not deliver the data until some threshold (or all
+	 * of it) is in place. So we must OR the nr_mapping_array and
+	 * mapping_array to get a true picture of the cum-ack.
 	 */
 	struct sctp_association *asoc;
 	int at;
+	uint8_t val;
 	int slide_from, slide_end, lgap, distance;
-
-	/* EY nr_mapping array variables */
-	/* int nr_at; */
-	/* int nr_last_all_ones = 0; */
-	/* int nr_slide_from, nr_slide_end, nr_lgap, nr_distance; */
 	uint32_t old_cumack, old_base, old_highest, highest_tsn;
 
 	asoc = &stcb->asoc;
@@ -2268,11 +2272,12 @@ sctp_slide_mapping_arrays(struct sctp_tcb *stcb)
 	 */
 	at = 0;
 	for (slide_from = 0; slide_from < stcb->asoc.mapping_array_size; slide_from++) {
-		if (asoc->nr_mapping_array[slide_from] == 0xff) {
+		val = asoc->nr_mapping_array[slide_from] | asoc->mapping_array[slide_from];
+		if (val == 0xff) {
 			at += 8;
 		} else {
 			/* there is a 0 bit */
-			at += sctp_map_lookup_tab[asoc->nr_mapping_array[slide_from]];
+			at += sctp_map_lookup_tab[val];
 			break;
 		}
 	}