diff --git a/lib/proto/types.c b/lib/proto/types.c
index d8b0c2cb1..7a2bf910a 100644
--- a/lib/proto/types.c
+++ b/lib/proto/types.c
@@ -305,7 +305,7 @@ ogs_amf_id_t *ogs_amf_id_build(ogs_amf_id_t *amf_id,
     return amf_id;
 }
 
-char *ogs_id_get_type(char *str)
+char *ogs_id_get_type(const char *str)
 {
     char *token, *p, *tmp;
     char *type = NULL;
@@ -335,7 +335,7 @@ cleanup:
     return type;
 }
 
-char *ogs_id_get_value(char *str)
+char *ogs_id_get_value(const char *str)
 {
     char *token, *p, *tmp;
     char *ueid = NULL;
diff --git a/lib/proto/types.h b/lib/proto/types.h
index 7ac37b9d2..bdab3d6ce 100644
--- a/lib/proto/types.h
+++ b/lib/proto/types.h
@@ -273,8 +273,8 @@ ogs_amf_id_t *ogs_amf_id_build(ogs_amf_id_t *amf_id,
 #define OGS_ID_SUPI_TYPE_IMSI "imsi"
 #define OGS_ID_GPSI_TYPE_MSISDN "msisdn"
 #define OGS_ID_SUPI_TYPE_IMEISV "imeisv"
-char *ogs_id_get_type(char *str);
-char *ogs_id_get_value(char *str);
+char *ogs_id_get_type(const char *str);
+char *ogs_id_get_value(const char *str);
 
 /************************************
  * TAI Structure                    */