diff --git a/gtests/net/packetdrill/run_packet.c b/gtests/net/packetdrill/run_packet.c
index 597a98ce6205d2dec19120f1308e4cf8e60fd624..27b3e66fab658553b9de466f5bb50a8d06b6aa28 100644
--- a/gtests/net/packetdrill/run_packet.c
+++ b/gtests/net/packetdrill/run_packet.c
@@ -617,7 +617,7 @@ static int map_inbound_sctp_packet(
 			nr_gap_blocks = ntohs(sack->nr_gap_blocks);
 			nr_dup_tsns = ntohs(sack->nr_dup_tsns);
 			for (i = 0; i < nr_dup_tsns; i++) {
-				sack->block[i].tsn = htonl(ntohl(sack->block[i].tsn) + local_diff);
+				sack->block[i + nr_gap_blocks].tsn = htonl(ntohl(sack->block[i + nr_gap_blocks].tsn) + local_diff);
 			}
 			break;
 		case SCTP_SHUTDOWN_CHUNK_TYPE: