diff --git a/include/osmocom/msc/vlr.h b/include/osmocom/msc/vlr.h
index 0b61a59..9943f15 100644
--- a/include/osmocom/msc/vlr.h
+++ b/include/osmocom/msc/vlr.h
@@ -284,7 +284,7 @@
 int vlr_subscr_rx_auth_resp(struct vlr_subscr *vsub, bool is_r99, bool is_utran,
 			    const uint8_t *res, uint8_t res_len);
 int vlr_subscr_rx_auth_fail(struct vlr_subscr *vsub, const uint8_t *auts);
-int vlr_subscr_tx_auth_fail_rep(struct vlr_subscr *vsub);
+int vlr_subscr_tx_auth_fail_rep(const struct vlr_subscr *vsub);
 void vlr_subscr_rx_ciph_res(struct vlr_subscr *vsub, struct vlr_ciph_result *res);
 int vlr_subscr_rx_tmsi_reall_compl(struct vlr_subscr *vsub);
 int vlr_subscr_rx_imsi_detach(struct vlr_subscr *vsub);
diff --git a/src/libvlr/vlr.c b/src/libvlr/vlr.c
index f57df8e..21ef082 100644
--- a/src/libvlr/vlr.c
+++ b/src/libvlr/vlr.c
@@ -125,8 +125,8 @@
 }
 
 /* Transmit GSUP message to HLR */
-static int vlr_tx_gsup_message(struct vlr_instance *vlr,
-			       struct osmo_gsup_message *gsup_msg)
+static int vlr_tx_gsup_message(const struct vlr_instance *vlr,
+			       const struct osmo_gsup_message *gsup_msg)
 {
 	struct msgb *msg = gsup_client_msgb_alloc();
 
@@ -146,7 +146,7 @@
 }
 
 /* Transmit GSUP message for subscriber to HLR, using IMSI from subscriber */
-static int vlr_subscr_tx_gsup_message(struct vlr_subscr *vsub,
+static int vlr_subscr_tx_gsup_message(const struct vlr_subscr *vsub,
 				      struct osmo_gsup_message *gsup_msg)
 {
 	struct vlr_instance *vlr = vsub->vlr;
@@ -158,7 +158,7 @@
 }
 
 /* Transmit GSUP error in response to original message */
-static int vlr_tx_gsup_error_reply(struct vlr_instance *vlr,
+static int vlr_tx_gsup_error_reply(const struct vlr_instance *vlr,
 				   struct osmo_gsup_message *gsup_orig,
 				   enum gsm48_gmm_cause cause)
 {
@@ -561,7 +561,7 @@
 }
 
 /* Tell HLR that authentication failure occurred */
-int vlr_subscr_tx_auth_fail_rep(struct vlr_subscr *vsub)
+int vlr_subscr_tx_auth_fail_rep(const struct vlr_subscr *vsub)
 {
 	struct osmo_gsup_message gsup_msg = {0};