diff --git a/glib/gutf8.c b/glib/gutf8.c
index 51cef3c19..b2a170b6f 100644
--- a/glib/gutf8.c
+++ b/glib/gutf8.c
@@ -1768,7 +1768,7 @@ fast_validate_len (const char *str,
  * g_utf8_validate:
  * @str: a pointer to character data
  * @max_len: max bytes to validate, or -1 to go until NUL
- * @end: return location for end of valid data
+ * @end: (allow-none) (out): return location for end of valid data
  * 
  * Validates UTF-8 encoded text. @str is the text to validate;
  * if @str is nul-terminated, then @max_len can be -1, otherwise