diff --git a/gio/gcontenttype.c b/gio/gcontenttype.c
index af8ef44c6..57249d94e 100644
--- a/gio/gcontenttype.c
+++ b/gio/gcontenttype.c
@@ -405,8 +405,8 @@ g_content_type_get_description (const gchar *type)
  *
  * Gets the mime type for the content type, if one is registered.
  *
- * Returns: (nullable): the registered mime type for the given @type,
- *     or %NULL if unknown.
+ * Returns: (nullable) (transfer full): the registered mime type for the
+ *     given @type, or %NULL if unknown; free with g_free().
  */
 char *
 g_content_type_get_mime_type (const char *type)