diff --git a/include/ntapi/nt_guid.h b/include/ntapi/nt_guid.h
index a091f42..2c27f23 100644
--- a/include/ntapi/nt_guid.h
+++ b/include/ntapi/nt_guid.h
@@ -31,7 +31,7 @@ typedef void __fastcall ntapi_tt_guid_to_string_utf16(
 	__out	nt_guid_str_utf16 *	guid_str);
 
 typedef int32_t __fastcall ntapi_tt_string_to_guid_utf16(
-	__in	nt_guid_str_utf16 *	guid_str,
-	__out	nt_guid *		guid);
+	__in	const nt_guid_str_utf16 *	guid_str,
+	__out	nt_guid *			guid);
 
 #endif
diff --git a/src/guid/ntapi_tt_guid.c b/src/guid/ntapi_tt_guid.c
index 69a5926..6f520fe 100644
--- a/src/guid/ntapi_tt_guid.c
+++ b/src/guid/ntapi_tt_guid.c
@@ -100,12 +100,12 @@ int32_t __fastcall __ntapi_tt_guid_compare(
 
 
 int32_t __fastcall __ntapi_tt_string_to_guid_utf16(
-	__out	nt_guid_str_utf16 *	guid_str,
-	__in	nt_guid *		guid)
+	__in	const nt_guid_str_utf16 *	guid_str,
+	__out	nt_guid *			guid)
 {
-	int32_t		status;
-	wchar16_t *	wch;
-	uint16_t	key;
+	int32_t			status;
+	const wchar16_t *	wch;
+	uint16_t		key;
 
 	/* outer braces / outer underscores / outer dashes */
 	if ((guid_str->lbrace == '{') && (guid_str->rbrace == '}'))