diff --git a/glib/gurifuncs.c b/glib/gurifuncs.c
index 5a4a71995..b2128a2f8 100644
--- a/glib/gurifuncs.c
+++ b/glib/gurifuncs.c
@@ -139,7 +139,8 @@ g_uri_unescape_segment (const char *escaped_string,
 /**
  * g_uri_unescape_string:
  * @escaped_string: an escaped string to be unescaped.
- * @illegal_characters: an optional string of illegal characters not to be allowed.
+ * @illegal_characters: (allow-none): a string of illegal characters not to be
+ *      allowed, or %NULL.
  * 
  * Unescapes a whole escaped string.
  *