diff --git a/src/osmo-bts-trx/sched_lchan_tchf.c b/src/osmo-bts-trx/sched_lchan_tchf.c
index b100fab35..1bf67a0e5 100644
--- a/src/osmo-bts-trx/sched_lchan_tchf.c
+++ b/src/osmo-bts-trx/sched_lchan_tchf.c
@@ -119,7 +119,7 @@ int rx_tchf_fn(struct l1sched_ts *l1ts, const struct trx_ul_burst_ind *bi)
 								: tch_mode) {
 	case GSM48_CMODE_SPEECH_V1: /* FR */
 		rc = gsm0503_tch_fr_decode(tch_data, *bursts_p, 1, 0, &n_errors, &n_bits_total);
-		if (rc >= 0)
+		if (rc == GSM_FR_BYTES) /* only for valid *speech* frames */
 			lchan_set_marker(osmo_fr_check_sid(tch_data, rc), lchan); /* DTXu */
 		break;
 	case GSM48_CMODE_SPEECH_EFR: /* EFR */
diff --git a/src/osmo-bts-trx/sched_lchan_tchh.c b/src/osmo-bts-trx/sched_lchan_tchh.c
index 51a0b90db..27880c129 100644
--- a/src/osmo-bts-trx/sched_lchan_tchh.c
+++ b/src/osmo-bts-trx/sched_lchan_tchh.c
@@ -147,8 +147,8 @@ int rx_tchh_fn(struct l1sched_ts *l1ts, const struct trx_ul_burst_ind *bi)
 		 */
 		rc = gsm0503_tch_hr_decode(tch_data, *bursts_p,
 			fn_is_odd, &n_errors, &n_bits_total);
-		if (rc >= 0) /* DTXu */
-			lchan_set_marker(osmo_hr_check_sid(tch_data, rc), lchan);
+		if (rc == (GSM_HR_BYTES + 1)) /* only for valid *speech* frames */
+			lchan_set_marker(osmo_hr_check_sid(tch_data, rc), lchan); /* DTXu */
 		break;
 	case GSM48_CMODE_SPEECH_AMR: /* AMR */
 		/* the first FN 0,8,17 or 1,9,18 defines that CMI is included