diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index f34d9ea..f2bf57b 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -890,7 +890,7 @@
 {
 	snprintf(buf, buf_len, "%06"PRIu32"/%02"PRIu16"/%02"PRIu8"/%02"PRIu8"/%02"PRIu8,
 		 tm->fn, tm->t1, tm->t2, tm->t3, (uint8_t)tm->fn%52);
-	buf[sizeof(buf)-1] = '\0';
+	buf[buf_len-1] = '\0';
 	return buf;
 }