diff --git a/lib/proto/types.c b/lib/proto/types.c
index adb2e3815..d8b0c2cb1 100644
--- a/lib/proto/types.c
+++ b/lib/proto/types.c
@@ -394,7 +394,7 @@ ogs_uint24_t ogs_s_nssai_sd_from_string(const char *hex)
     return ogs_uint24_from_string((char *)hex);
 }
 
-int ogs_fqdn_build(char *dst, char *src, int length)
+int ogs_fqdn_build(char *dst, const char *src, int length)
 {
     int i = 0, j = 0;
 
@@ -411,7 +411,7 @@ int ogs_fqdn_build(char *dst, char *src, int length)
     return length+1;
 }
 
-int ogs_fqdn_parse(char *dst, char *src, int length)
+int ogs_fqdn_parse(char *dst, const char *src, int length)
 {
     int i = 0, j = 0;
     uint8_t len = 0;
diff --git a/lib/proto/types.h b/lib/proto/types.h
index 78b013a15..7ac37b9d2 100644
--- a/lib/proto/types.h
+++ b/lib/proto/types.h
@@ -574,8 +574,8 @@ typedef struct ogs_session_s {
     ogs_ip_t smf_ip;
 } ogs_session_t;
 
-int ogs_fqdn_build(char *dst, char *src, int len);
-int ogs_fqdn_parse(char *dst, char *src, int len);
+int ogs_fqdn_build(char *dst, const char *src, int len);
+int ogs_fqdn_parse(char *dst, const char *src, int len);
 
 /**************************************************
  * Protocol Configuration Options Structure