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";