diff --git a/src/gsm/gsm0808_utils.c b/src/gsm/gsm0808_utils.c
index 4993539f3..11e86b8a1 100644
--- a/src/gsm/gsm0808_utils.c
+++ b/src/gsm/gsm0808_utils.c
@@ -541,6 +541,7 @@ uint8_t gsm0808_enc_channel_type(struct msgb *msg,
 		break;
 	case GSM0808_CHAN_SIGN:
 		/* Octet 5 is spare */
+		msgb_put_u8(msg, 0);
 		break;
 	default:
 		OSMO_ASSERT(false);