diff --git a/openbsc/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index 1fb80a2..ad6401e 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -388,7 +388,7 @@
 void set_ts_e1link(struct gsm_bts_trx_ts *ts, u_int8_t e1_nr,
 		   u_int8_t e1_ts, u_int8_t e1_ts_ss);
 enum gsm_bts_type parse_btstype(char *arg);
-char *btstype2str(enum gsm_bts_type type);
+const char *btstype2str(enum gsm_bts_type type);
 struct gsm_bts *gsm_bts_by_lac(struct gsm_network *net, unsigned int lac,
 				struct gsm_bts *start_bts);
 
diff --git a/openbsc/src/gsm_data.c b/openbsc/src/gsm_data.c
index a78425f..e5e789a 100644
--- a/openbsc/src/gsm_data.c
+++ b/openbsc/src/gsm_data.c
@@ -174,7 +174,7 @@
 	return GSM_BTS_TYPE_BS11; /* Default: BS11 */
 }
 
-char *btstype2str(enum gsm_bts_type type)
+const char *btstype2str(enum gsm_bts_type type)
 {
 	if (type > ARRAY_SIZE(bts_types))
 		return "undefined";