It is no a bug, you need to put object id (number) (no object name).
To get id of object you can use wowhead, for Silkweed the wowhead links is http://www.wowhead.com/object=209350 and http://www.wowhead.com/object=221544 (id is in url: 209350 and 221544).
ps: You need to put object id, not item id (check if wowhead url is finish by object=ID not by item=ID), or you can also use dev tools for get id (entry).