diff --git a/openbsc/src/abis_rsl.c b/openbsc/src/abis_rsl.c
index 8a4ed9b..d5c883e 100644
--- a/openbsc/src/abis_rsl.c
+++ b/openbsc/src/abis_rsl.c
@@ -241,7 +241,7 @@
 }
 
 /* See Table 10.5.25 of GSM04.08 */
-u_int8_t lchan2chan_nr(struct gsm_lchan *lchan)
+u_int8_t lchan2chan_nr(const struct gsm_lchan *lchan)
 {
 	struct gsm_bts_trx_ts *ts = lchan->ts;
 	u_int8_t cbits, chan_nr;