diff --git a/src/osmo-bts-trx/sched_lchan_tchf.c b/src/osmo-bts-trx/sched_lchan_tchf.c
index 6a6331f76..eff0f63a7 100644
--- a/src/osmo-bts-trx/sched_lchan_tchf.c
+++ b/src/osmo-bts-trx/sched_lchan_tchf.c
@@ -412,7 +412,7 @@ struct msgb *tch_dl_dequeue(struct l1sched_ts *l1ts, struct trx_dl_burst_req *br
 		switch (tch_mode) {
 		case GSM48_CMODE_SPEECH_V1: /* FR / HR */
 			if (br->chan != TRXC_TCHF) /* HR */
-				len = 15;
+				len = GSM_HR_BYTES + 1; /* RFC 5993 */
 			else
 				len = GSM_FR_BYTES;
 			break;