diff --git a/include/osmocom/sgsn/gprs_sgsn.h b/include/osmocom/sgsn/gprs_sgsn.h
index cf78766..0b721a3 100644
--- a/include/osmocom/sgsn/gprs_sgsn.h
+++ b/include/osmocom/sgsn/gprs_sgsn.h
@@ -460,7 +460,7 @@
 extern const struct value_string *sgsn_auth_state_names;
 
 void sgsn_auth_init(void);
-struct imsi_acl_entry *sgsn_acl_lookup(const char *imsi, struct sgsn_config *cfg);
+struct imsi_acl_entry *sgsn_acl_lookup(const char *imsi, const struct sgsn_config *cfg);
 int sgsn_acl_add(const char *imsi, struct sgsn_config *cfg);
 int sgsn_acl_del(const char *imsi, struct sgsn_config *cfg);
 /* Request authorization */